Preheat your oven to 400°F.
Cut the tops off the bell peppers and scoop out the seeds and stems.
Place bell peppers in an oven-proof dish and bake for 15 minutes, then reduce oven to 350°F.
Sauté onions in 1 tbsp olive oil until golden brown, then set aside.
Brown the ground beef in the same skillet until fully cooked.
Set aside 1/3 cup (80mls) of tomato sauce for topping.
Mix cooked ground beef, onions, Italian seasoning, parsley, salt, pepper, mushrooms, garlic, Worcestershire sauce, and 2/3 cup tomato sauce.
Pour out any liquid from the pre-cooked peppers before stuffing them.