Slice sirloin into 1-inch cubes, then pat completely dry with paper towels for 2 minutes. Season all sides evenly with salt and black pepper.
1.5 lb sirloin steak, 1 tsp salt, 0.5 tsp black pepper
Heat cast iron skillet over medium-high heat for 5 minutes until a drop of water sizzles instantly. Add olive oil and swirl to coat the surface.
2 tbsp olive oil
Arrange steak cubes in a single layer, at least 1 inch apart. Cook in 2 batches if needed to avoid overcrowding. Sear 2-3 minutes per side until a golden brown crust forms. Transfer seared steak to a plate.
1.5 lb sirloin steak
Lower heat to medium. Add minced garlic to the empty pan and cook for 30 seconds until fragrant. Add unsalted butter and stir until melted.
3 tbsp unsalted butter, 4 cloves garlic
Return steak bites to the pan and toss in the garlic butter for 1 minute, spooning the sauce continuously over the top. Remove from heat, cover loosely with foil, and rest for 5 minutes before serving. Add fresh parsley and optional red pepper flakes and lemon juice before plating.
1.5 lb sirloin steak, 3 tbsp unsalted butter, 4 cloves garlic, 2 tbsp fresh parsley, 0.25 tsp red pepper flakes, 1 tbsp lemon juice
