Pat the steak cubes dry with paper towels. Season generously with salt, pepper, and Italian seasoning if using.
Heat 1 tablespoon butter and the olive oil in a large skillet over medium-high heat. Add the steak bites in a single layer (work in batches if needed). Sear for about 2–3 minutes per side until golden brown but still tender. Remove steak bites from the skillet and set aside.
In the same skillet, add 1 tablespoon butter and the sliced mushrooms. Cook for 5–6 minutes, stirring occasionally, until mushrooms are browned and tender.
Lower the heat to medium. Stir in the minced garlic and cook for about 30 seconds until fragrant. Return the steak bites to the pan along with the remaining 1 tablespoon of butter. Toss everything together until coated and heated through.
Garnish with fresh chopped parsley and serve immediately.