In a 1-quart round baking dish, drizzle the 1/4 cup balsamic vinaigrette over mushrooms; toss to coat. Cover and marinate in the refrigerator for 2 hours. Bake mushroom mixture, uncovered, in a 375 degree F oven for 20 minutes. Remove from oven.
Slice rolls almost all the way through. Wrap the uncut sides of rolls with foil, so rolls stand with cut sides facing up. Fill with mushroom mixture and cheese. Arrange sandwiches in foil, mushroom-filled side up, on a baking sheet. Bake for 5 minutes.
To serve, top each with mesclun and drizzle with the 2 tablespoons bottled balsamic vinaigrette. Top with tomato slices. Makes 2 sandwiches.