Pat the steak dry. Moisture prevents browning. Use paper towels to dry the cubes well. This one step makes a big difference.
Season generously. Toss the steak with 1–1.5 teaspoons kosher salt, 1/2 teaspoon black pepper, and a light sprinkle of garlic powder if you like.
Seasoning should be bold but not brash.
Preheat your pan. Set a large stainless steel or cast-iron skillet over medium-high heat. Add 1 tablespoon oil and heat until it shimmers. A hot pan equals a great sear.
Sear in batches. Add half the steak cubes in a single layer with a little space between them.
Don’t crowd the pan. Let them sear, untouched, for about 1.5–2 minutes until a deep brown crust forms, then flip and cook another 1–2 minutes. Transfer to a plate.
Repeat with remaining oil and steak.
Lower the heat and make garlic butter. Reduce heat to medium-low. Add butter to the pan. When it foams, stir in the minced garlic.
Cook 30–45 seconds, just until fragrant. Don’t brown the garlic.
Toss and finish. Return all steak to the pan. Toss in the garlic butter for 30–60 seconds.
Add a pinch of red pepper flakes if using. Remove from heat and stir in 1–2 teaspoons lemon juice and a little lemon zest.
Add Parmesan and herbs. Sprinkle the Parmesan and chopped parsley over the steak bites. Toss gently to coat.
The cheese will cling to the buttery crust without turning gummy.
Taste and adjust. Finish with a final pinch of salt and pepper if needed. Serve immediately with extra Parmesan and a wedge of lemon on the side.