01 - Pat steak cubes dry with paper towels, then season evenly with kosher salt and freshly ground black pepper.
02 - Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add mushrooms and sauté for 4 to 5 minutes, stirring occasionally, until browned and tender. Remove mushrooms and set aside.
03 - Add remaining 1 tablespoon olive oil and 1 tablespoon butter to the skillet. When hot, add steak bites in a single layer, working in batches if needed. Sear without stirring for 2 to 3 minutes, then flip and cook another 2 minutes until browned and cooked to desired doneness.
04 - Add remaining 2 tablespoons butter and minced garlic to the skillet. Stir and cook for 1 minute until fragrant.
05 - Return mushrooms to the skillet. Add chopped parsley, dried thyme, and chili flakes if using. Toss everything to coat evenly with garlic butter sauce.
06 - Plate immediately and garnish with additional parsley if desired.