Preheat your oven to 375°F (190°C).

In a large skillet over medium heat, melt 2 tablespoons of butter. Add 1 diced onion and 1 cup of chopped mushrooms, sautéing until the onions become translucent, about 5 minutes. Then, add 1 pound of ground beef and cook until browned. Stir in 2 tablespoons of Worcestershire sauce, 1 cup of crushed tomatoes, and season with salt and black pepper to taste.

Slice the tops off of 4 bell peppers and remove the seeds. Stand the peppers upright in a baking dish. Spoon the beef mixture into each pepper, packing the filling down slightly.

Bake for 25-30 minutes until the peppers are tender and slightly blistered.

Remove from oven and sprinkle with 1 cup of shredded mozzarella cheese. Return to the oven for an additional 5-10 minutes until the cheese is melted and bubbly.

Let the peppers cool for a few minutes, then serve hot.