You can prepare this the day before serving, then refrigerate and bake when needed.
3 large Vidalia onions, sliced at least 1/4-inch thick
1/2 to 1 pound fresh mushrooms, sliced
1 can cream of mushroom soup
1 small can evaporated milk
1 tablespoon soy sauce
8 ounces or more shredded Swiss cheese
1 large loaf French bread, sliced