In large bowl, toss onion, pepper, mushrooms, 2 tablespoons of the vegetable oil and 1 tablespoon of the steak seasoning; mix until evenly coated. Transfer vegetable mixture to sheet pan in single layer, allowing room for steak. In same bowl, add remaining 2 tablespoons oil and remaining 1 tablespoon steak seasoning. Add steak; rub mixture into steak. Transfer steak to pan with vegetables.