Preheat the oven to 400 degrees. Line a baking sheet with parchment paper and set aside. Prepare 2 large green bell peppers by slicing them in half and removing the membrane.
Add 2 tablespoons salted butter, 2 tablespoons olive oil, 1 tablespoon minced garlic, ¼ teaspoon salt, and ⅙ teaspoon black pepper to a large skillet and cook over medium heat for about 1-2 minutes, until the garlic is fragrant.
Add 1 cup diced sweet onion, and 8 ounces sliced baby bella mushrooms and cook for about 10 minutes, until the onions are caramelizing and the mushrooms have released all their liquid.
Add 8 ounces thinly sliced roast beef to the skillet and stir to combine. Cook for 5 minutes, or until all the meat is heated through.
Line the bottom of each half of the bell peppers with a slice of 8 slices provolone cheese. Fill the peppers until they are full and rounded on top with the roast beef mixture. Add another slice of cheese on top.
Bake for 15-20 minutes, or until the cheese is nice and bubbly and the peppers have started to soften. Serve immediately.