Preheat the oven to 375°F (190°C).

Cut the tops off the bell peppers and remove the seeds and membranes. Set them aside.

In a large skillet, heat the olive oil over medium heat. Add the ground beef (or plant-based alternative) and cook until browned, breaking it apart with a spoon as it cooks. Drain any excess fat.

Add the chopped mushrooms, onions (if using), and garlic (if using) to the skillet. Cook until the mushrooms are softened and any moisture from the mushrooms has evaporated.

Stir in the BBQ sauce and cook for another 2-3 minutes, allowing the flavors to combine.

Stuff each bell pepper with the beef and mushroom mixture, packing it gently.

Top each stuffed pepper with shredded cheese.

Place the stuffed peppers in a baking dish and bake for 20-25 minutes, or until the peppers are tender and the cheese is melted and bubbly.

Remove from the oven and let the peppers cool slightly before serving.