Preheat oven to 400°.

Slice the peppers in half lengthwise and remove ribs and seeds.

Place peppers in the oven to soften for 20 minutes.

Add 1 Tbsp of butter and 1 Tbsp of olive oil to a large saute' pan over medium heat. Cook the sliced beef stirring frequently, until browned. Set aside.

Add remaining 1 Tbsp butter, 1 Tbsp olive oil, garlic, mushrooms, onions and salt and pepper to the pan and cook over low-medium heat until tender.

Add the beef to the onion & mushroom mixture. Cook for 3-5 minutes until heated through.

Line the inside of each pepper with a slice of cheese and fill with the meat mixture and top with another slice of cheese.

Bake for 15-20 minutes until the cheese on top is golden brown. Broil for final 2 minutes to get the cheese to bubble.