Pat steak cubes thoroughly dry with paper towels (moisture prevents a crisp sear). Season all sides evenly with kosher salt, cracked black pepper, and garlic powder.

Heat 1 ½ tablespoons of oil in a large 12-inch cast-iron skillet over high heat until just beginning to lightly smoke.

Add steak cubes in a single, uncrowded layer (sear in two batches if necessary). Sear undisturbed for 2 minutes to develop a deep golden-brown crust, then flip and sear for another 1 to 2 minutes until medium-rare to medium (130°F–135°F / 55°C). Transfer seared steak bites to a plate and tent loosely with foil.

Reduce skillet heat to medium-high. Add remaining ½ tablespoon oil and 1 tablespoon of butter. Add mushrooms cut-side down. Cook undisturbed for 3 minutes until deep golden-brown, then stir and cook 2 minutes more.

Pour in the beef bone broth and soy sauce, scraping up all the savory browned bits (fond) from the pan bottom. Simmer for 1 minute until liquid reduces by half.

Reduce heat to medium-low. Add remaining 3 tablespoons of butter, minced garlic, rosemary sprigs, and thyme. Swirl and spoon the foaming garlic herb butter over the mushrooms for 60 seconds until garlic is fragrant and sweet.

Toss the seared steak bites and any resting juices back into the skillet. Swirl rapidly for 30 to 45 seconds until the steak cubes are coated in glistening garlic butter. Remove from heat, discard herb sprigs, sprinkle with fresh parsley, and serve immediately straight from the skillet.