Pat the steak dry, cut into 1-inch cubes, and season all over with salt and pepper.
Heat a large skillet over medium-high. Add the olive oil and swirl to coat.
Sear the steak bites in a single layer, working in batches. Cook 1–2 minutes per side until well browned. Transfer to a plate.
Lower heat to medium. Add the butter to the pan. Stir in the garlic and cook until fragrant, about 30 seconds.
Return steak to the skillet and toss in the garlic butter for 30–60 seconds. Stir in parsley, red pepper flakes, and lemon juice.
Taste and adjust seasoning. Serve hot, spooning extra garlic butter over the top.