For stuffing, in a small saucepan cook the onion and garlic in margarine or butter until onion is tender but not brown. Remove from heat. Stir in the lemon-pepper seasoning or pepper. Set aside.
Make a pocket in the steak by cutting a deep horizontal slit. Spoon the stuffing into the pocket. If necessary, secure opening of the pocket with a wooden toothpick.
To grill, place steak on the rack of an uncovered grill. Grill directly over medium-hot coals to desired doneness, turning once. (Allow 14 to 18 minutes for rare, 18 to 22 minutes for medium, or 24 to 28 minutes for well-done).
Or, to broil, place steak on the unheated rack of a broiler pan. Broil 4 to 5 inches from the heat to desired doneness, turning once. (Allow 14 to 18 minutes for rare, 19 to 22 minutes for medium, or 23 to 28 minutes for well-done).
Meanwhile, for mushroom sauce, in a small saucepan cook the mushrooms in margarine or butter until tender. In a small bowl stir together the cornstarch, Worcestershire sauce, bouillon granules, and 1/2 cup water. Carefully add to the saucepan. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Stir in wine. To serve, cut steak in half. Transfer steak halves to individual plates. Pour sauce over steak. Makes 2 servings.
Grill new potatoes to round out the meal.