Heat oven to 375°F.
Spray 13x9 glass or ceramic baking dish with non-stick spray.
In large skillet, brown ground beef, onion, and bell pepper stirring occasionally, until browned.
Drain off any grease.
Stir in garlic, mushrooms and oregano.
Stir in half of the pepperoni slices and 1 cup of the sauce.
Cook 5 minutes or until heated through. Remove from heat.
While the beef browns, cook the pasta shells as directed on package and drain.
Stuff each shell with 2 tablespoons beef mixture and place in a single layer in the baking dish.
Cover the shells with remaining sauce, shredded cheese and the remaining half of the pepperoni.
Bake 25 minutes or until the cheese is melted and bubbly.
Remove from oven and let stand 5 minutes before serving.