Prebake pie crusts until light golden brown according to package instructions.

Brown Italian sausage in a skillet.

Chop green bell pepper, white onion, and mushrooms.

Brown half of the chopped vegetables with the sausage.

Remove pie crusts from oven and let them cool.

Cool the sausage mixture.

In a large mixing bowl, combine all of the vegetables, sausage, and most of the shredded mozzarella cheese (reserve some for topping).

Add spaghetti sauce to the mixture until it is moist but not sloppy. Aim for a consistency similar to fruit pie filling.

Fill each prebaked pie crust with the sausage and vegetable filling.

Top each pie with the reserved mozzarella cheese and sliced pepperoni.

Bake at 300°F (149°C) until the cheese melts and the pies are warmed through, approximately 30-40 minutes.