Preheat oven to 375°F. Line two baking sheets with parchment paper and grease lightly.
Slice the onions into very thin rounds, about 1/4-inch thick (you'll need 16 slices total).
Arrange 8 small piles of shredded cheddar cheese on each baking sheet, spacing a few inches apart.
Place one onion round on top of each pile of cheese. Sprinkle evenly with ranch seasoning.
Bake for 11-13 minutes, watching closely towards the end so they don't burn. They're ready when crispy and lightly browned.
Let cool for 5-7 minutes before removing crisps from baking sheets.
Serve warm with ranch or marinara sauce for dipping.