In a large skillet, brown ground beef and onion in vegetable oil until done. Season with salt, pepper, parsley flakes, and Worcestershire sauce. Add tomato soup and tomato sauce. Simmer for about 15 minutes. Add cooked rice.
Cut a thin slice from the stem end of bell peppers and remove seeds. Simmer peppers in a large pan of boiling water with 1/2 teaspoon salt and 1/2 teaspoon sugar for three to five minutes. Remove from water; drain and arrange in a large baking dish with cut ends up.
Stuff peppers with meat/rice mixture. Top with sliced mushrooms. Bake at 350ยบ for about 30 minutes. Remove from oven and top with Mozzarella cheese. Return to oven and bake until cheese is melted.