Philly Cheesesteak Stuffed Peppers (Low-Carb, Keto)

Name: Philly Cheesesteak Stuffed Peppers (Low-Carb, Keto)
Ingredients: 2 bell peppers, any color you like; 1 tbsp olive oil; 1/2 large yellow onion, sliced; 4 oz mushrooms, sliced; 8 slices thinly-sliced deli roast beef, no sugar added, chopped into thin strips; 1/2 tsp garlic powder; salt and pepper, to taste; 8 slices provolone cheese