4 Portobello caps
1 medium onion, sliced
1 pound ground beef (or pork, lamb, or shredded meat)
Salt and pepper to taste
3/4 cup commercial barbecue sauce
4 slices mozzarella or Monterey Jack cheese
Preheat the oven to 350*F (175*C).
Brush the mushroom caps with oil, place on baking sheet and bake for 2 minutes.
Meanwhile, heat 1 tablespoon oil in medium skillet and saute the onion for 1 minute. Add the ground beef and cook until the meat is done; drain. Season with salt and pepper to taste. Stir in the barbecue sauce.
Remove the caps from the oven and cover with the meat mixture. Cover each with a slice of cheese and return to the oven until cheese melts, about 1 minute. Serve immediately.