Sauté a mix of mushrooms in butter or oil with garlic, thyme, salt, and pepper until deeply browned. Deglaze pan with a splash of balsamic or soy to boost umami. Pile mushrooms on bread with melty cheese like provolone, Swiss, or fontina and grill until cheese melts.
Assembly pointers:
This sandwich is hearty, savory, and perfect as a vegetarian main that still feels indulgent.