Cut 1 1/2 pounds sirloin steak into bite-sized pieces. Pat them very dry with paper towels.
Heat a large skillet (cast iron works great) over high heat. Add 1 Tablespoon olive oil and let it get very hot.
Season steak pieces generously with 1/2 teaspoon salt and 1/2 teaspoon black pepper.
Add steak to the hot pan in a single layer. Don't stir! Let it cook undisturbed for at least 2 minutes to get a good sear.
Flip the steak pieces and cook for another 2 minutes until golden brown. If your pan isn't big enough, work in batches so the steak isn't crowded.
Remove steak from the pan and set on a plate.
Turn heat down to medium. Add 2 Tablespoons unsalted butter to the same pan.
Once butter is melted, add 4 cloves garlic, minced and 1/4 teaspoon red pepper flakes (if using). Cook for about 30 seconds, stirring constantly, just until garlic becomes fragrant. Don't let the garlic burn.
Pour the garlic butter over the steak bites and toss to coat.
Garnish with 1 Tablespoon fresh parsley, chopped (optional) and serve immediately.