Bacon and Caramelized Onion Grilled Cheese

This Bacon and Caramelized Onion Grilled Cheese is the ultimate grown-up toastie – crispy bacon, sweet jammy onions, and a gorgeous blend of melted cheddar and Gruyere between perfectly golden bread!

If you love proper comfort food like my Philly Cheesesteak Casserole and Loaded Bacon Cheeseburger Pasta, this elevated grilled cheese is going to become your new favourite lunch or quick dinner!

I’ve always loved a good cheese toastie, but this version takes it to another level entirely. The caramelized onions add this incredible sweetness that perfectly balances the salty bacon and rich cheese. Yes, the onions take about 40 minutes to cook, but honestly, they’re mostly hands-off and the result is absolutely worth it. Plus, cooking them in the bacon fat adds so much depth of flavour.

What makes this sandwich special is the combination of cheeses – sharp cheddar gives you that classic grilled cheese taste, whilst Gruyere adds these gorgeous nutty notes and melts beautifully. The mayo on the outside (trust me on this!) creates the most perfect golden, crispy crust. It’s one of those sandwiches where you take a bite and everything just works together brilliantly.

Why You’re Going To Love This Recipe

Next-level grilled cheese – this is not your average toastie!
Sweet and savoury – caramelized onions balance the bacon perfectly.
Crispy bacon fat onions – cooking them in bacon fat is genius.
Perfect cheese blend – cheddar and Gruyere melt beautifully together.
Golden crispy exterior – mayo creates the best crust.
Comfort food elevated – fancy enough for guests, easy enough for everyday.

Jump to:

Why You’re Going To Love This Recipe
What You Need
Making The Perfect Sandwich
Handy Tips
Questions You Might Have
More Brilliant Sandwiches

What You Need

Simple ingredients create this spectacular sandwich!

Note: Full measurements and complete instructions are in the recipe card at the bottom.

Bacon – about 8 strips. We’ll use the bacon fat to cook the onions!
Yellow onions – 2 large ones, thinly sliced. White onions work too, but yellow are sweeter.
Sharp white cheddar – 1 cup shredded. The sharp flavour is brilliant here.
Gruyere cheese – 1 cup shredded. Adds nutty, complex flavour and melts gorgeously.
Mayo or butter – 4 tablespoons for spreading on the bread. I’m team mayo for the crispiest crust!
Bread – 8 slices of something sturdy. Sourdough is my favourite, but wholemeal or white work well.
Salt and pepper – for seasoning the onions.

Making The Perfect Sandwich

This comes together in straightforward steps!

Note: Pop down to the recipe card for exact quantities and the complete method.

Cook the bacon

Start with a cold frying pan and lay your bacon strips in it. Turn the heat to medium and cook until crispy, about 5-6 minutes per side. Remove to a paper towel-lined plate to drain. If your pan isn’t big enough, do this in batches.

Caramelize the onions

This is where the magic happens! Pour off all but 2 tablespoons of bacon fat from your pan. Add your thinly sliced onions over medium-low heat and stir to coat them in that lovely bacon fat. Now comes the patient bit – cook these onions, stirring occasionally, for 30-40 minutes. You want them soft, jammy, and a gorgeous golden brown colour. If they’re browning too quickly, turn the heat down. If anything starts to stick or burn, add a splash of water. Season with salt and pepper. Once they’re done, set them aside.

Build your sandwich

Wipe your pan clean with kitchen paper. Spread mayo (or butter) on both sides of two slices of bread. Place one slice mayo-side down in your cold pan. Layer on about half a cup of cheese (mix of cheddar and Gruyere), some caramelized onions, and a couple of bacon strips. If some cheese escapes onto the pan, that’s brilliant – it’ll create crispy cheese bits! Top with your second slice of bread, mayo-side up.

Cook it perfectly

Turn your heat to medium. Cook for 3-4 minutes until the bottom is golden brown, pressing down gently with a spatula. Flip it over carefully, then pop a lid on top – this helps steam and melt the cheese properly. Cook for another 3-4 minutes until golden and the cheese is completely melted. Repeat with remaining ingredients.

Serve

Transfer to a cutting board, slice in half (so you can see all those gorgeous melty layers!), and serve while hot.

Handy Tips

Little tricks to make the ultimate grilled cheese:

Patience with onions – don’t rush them! Proper caramelization takes 30-40 minutes.
Save that bacon fat – it adds so much flavour to the onions.
Slice onions thinly – they’ll caramelize more evenly.
Use a lid – covering while cooking helps melt the cheese perfectly.
Mayo vs butter debate – mayo creates a crispier crust, butter tastes richer. Both work!
Don’t overload – too much filling makes flipping difficult.
Press gently – helps everything stick together but don’t squash it flat.
Grate your own cheese – pre-shredded doesn’t melt as well.
Make onions ahead – caramelize a big batch and keep in the fridge for up to a week.
Add herbs – fresh thyme or crispy sage are lovely additions.

Questions You Might Have

Can I use different cheeses?

Absolutely! Fontina, Gouda, Parmesan, or even good old American cheese all work. Just make sure whatever you choose melts well.

I don’t have time to caramelize onions – any shortcuts?

You could use shop-bought caramelized onions, or cook them ahead and keep them in the fridge. They’re brilliant on so many things, so it’s worth making a big batch!

Mayo or butter – which is better?

It’s personal preference! Mayo creates a crispier, more golden crust, whilst butter gives a richer flavour. Both are delicious.

Can I make these in advance?

The onions can be made ahead and refrigerated for up to a week. The bacon can be cooked earlier too. But assemble and cook the sandwiches fresh for the best texture.

What else can I do with leftover caramelized onions?

They’re brilliant on pizza, mixed into pasta, topping burgers, stirred through mashed potatoes, or even just on toast with cheese. French onion soup is another favourite!

My cheese isn’t melting properly – help!

Make sure you’re using the lid technique and keeping the heat at medium (not too high). Also, freshly grated cheese melts much better than pre-shredded.

Can I add other vegetables?

Sliced tomatoes or sautéed mushrooms would be lovely! Just make sure any additions aren’t too wet or they’ll make the bread soggy.

More Brilliant Sandwiches

Other satisfying handheld meals perfect for lunch or dinner:

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Bacon and Caramelized Onion Grilled Cheese

Crispy bacon, sweet jammy onions, and melted cheddar and Gruyere between perfectly golden bread – the ultimate grown-up grilled cheese sandwich!

Course: Lunch, Dinner
Cuisine: American
Prep Time: 10 minutes
Cook Time: 50 minutes
Total Time: 1 hour
Serves: 4 sandwiches

Ingredients

8 oz bacon (about 8 strips)
2 large yellow onions, thinly sliced (or 4 small)
Salt and black pepper, to taste
1 cup sharp white cheddar cheese, shredded
1 cup Gruyere cheese, shredded
4 tablespoons mayo or softened butter
8 slices bread (sourdough, wholemeal, or white)

Instructions

Place bacon strips in a cold large frying pan. Turn heat to medium and cook until crispy, about 5-6 minutes per side. Remove to a paper towel-lined plate. Do this in batches if needed.
Pour off all but 2 tablespoons of bacon fat from the pan. Add the sliced onions over medium-low heat and stir to coat in the bacon fat.
Cook the onions, stirring occasionally, until soft, jammy, and translucent, about 30-40 minutes. If browning too fast, reduce heat to low. Season with salt and pepper.
Continue cooking until onions are rich golden brown and super jammy. If anything sticks or burns, add a splash of water. Once done, set aside in a bowl.
Wipe the pan clean with kitchen paper. Spread mayo (or butter) on both sides of 2 slices of bread.
Place one slice mayo-side down in the cold pan. Top with about ½ cup mixed cheese, some caramelized onions, and 2 bacon strips. Top with the second slice of bread, mayo-side up.
Turn heat to medium. Cook for 3-4 minutes until golden brown, pressing gently with a spatula.
Flip carefully, cover with a lid, and cook for another 3-4 minutes until golden and cheese is melted.
Repeat with remaining ingredients. Transfer to a cutting board, slice in half, and serve hot!