These delectable vegetarian morsels are hearty enough for a main dish and delicate enough for a side or even an appetizer.
Preheat the oven to 375 degrees F.
Put the mushroom caps into a lightly oiled baking pan that fits them snugly, then spoon the filling mixture into the mushroom caps.
Bake them for 20 minutes (30 minutes if you prepared the caps ahead and refrigerated them) or until the bread crumbs and cheese on top begins to brown. Serve immediately.
The ratio of filling to mushroom caps is not exact. If you have filling left over, you can use it to fill a hollowed-out Campari (medium-sized) or larger tomato or a zucchini, halved and hollowed out.
The mushrooms can be prepared a day in advance and refrigerated, covered in foil, until ready to bake. If the caps have been refrigerated, the baking time will be longer, and you may want to leave the foil covering on for the first 10 minutes of baking time.