1. Heat grill for direct grilling.
2. Sprinkle both sides of steak with the 1/2 teaspoon salt and the 1/4 teaspoon pepper. Grill steak over medium* heat 6 to 7 minutes per side or until instant-read thermometer inserted into thickest side of steak registers 140 degrees F. Transfer to cutting board and let stand 5 minutes before slicing. (Internal temperature will increase to 145 degrees F. for medium-rare.)Make Mushroom Topping::
3. Meanwhile, heat oil in skillet over medium-high heat; add shallots and cook, stirring, 2 minutes. Stir in garlic; cook 30 seconds. Remove half mixture to cup; set aside. Add mushrooms, the 1/4 teaspoon salt and the 1/8 teaspoon pepper to skillet; cook 6 to 7 minutes or until golden.
4. Thinly slice steak. Spread bottom half of bread with shallot mixture. Arrange steak slices on top. Spoon on mushrooms, then divide and dot cheese over mushrooms. Cover with top half of bread and cut crosswise into 4 sandwiches. Makes 4 servings.