Sauté ground beef. Mix cottage or ricotta cheese, butter, and tomato sauce until well blended.
Stir in mushrooms and green pepper, blending well. Add oregano and pepper.
Place layer of cooked shells in bottom of lightly buttered casserole. Cover with layer of cottage cheese mixture. Top with half of the pepperoni. Place 2 slices Mozzarella cheese on top. Crumble 1/3 ground beef evenly over cheese.
Repeat layers until all ingredients are used, ending with Mozzarella cheese on top, ringed with pepperoni slices.
Bake at 350 degrees F for 45 minutes until hot and bubbly.