Cook the ground beef in a skillet over medium heat until cooked through. Season lightly with salt and pepper. Set aside.
Prepare the mushrooms by wiping off the caps and removing all of the stems.
Line the mushrooms up in a baking dish, cap side down.
Fill each mushroom with a combination of the ground beef, some bell pepper, and some onion.
Sprinkle each mushroom with the mozzarella cheese or cut the provolone slices into pieces and lay them on top of the mushrooms.
Bake the mushrooms for 10-15 minutes until the mushrooms are tender. Bake longer if you want softer mushrooms, however they will become more difficult to eat by hand. Serve immediately.
If you want your mushrooms browned on top but it is not happening, pop them under the broiler in the oven for just a minute or two. Make sure they are NOT on a silicone baking mat if you do this as it will blister and burn. Just put them on a metal pan or in a baking dish.You may need more or less of the sauce and cheeses depending on exactly how large the mushrooms are. Fill with each ingredient to your desired preference.To ensure keto, use a keto marinara.