Preheat your oven to 400 degrees F and grease a 9×12 casserole baking dish.
Cut the tops off of the peppers and remove the center seeds and membranes. Place the peppers in the baking dish, standing up with the hollowed side facing upward.
Place the olive oil in a large skillet and saute the onions and garlic until soft, about 3 minutes.
Add the ground beef to the skillet and cook until no longer pink, about 8-10 minutes. Chop the beef with a spatula as it cooks so that it is in small pieces.
Add the chili powder, cumin, salt, pepper and tomatoes to the pan and stir well. Simmer the mix for 5 minutes.
Turn the heat off and then stir in half of the cheese.
Scoop the beef mix into the peppers in the pan, stuffing them equally. Sprinkle the tops of the peppers with the remaining cheese
Bake the peppers in the oven for 20-25 minutes or until the cheese on the top has melted and is beginning to brown. Serve immediately with a dollop of sour cream.