Heat the skillet on the stovetop until hot. Add olive oil – it should sizzle. Immediately add ground beef and cook it, constantly stirring, until nicely browned and cooked through. Remove from heat. Drain all excess fat. Add 1 teaspoon chili powder, 1 teaspoon cumin, 1/4 teaspoon salt to cooked ground beef, and stir to combine.
To the same skillet, off heat, add black beans, chopped tomatoes, and cooked rice. Note: make sure to rinse black beans really well and drain. Add more spices: 1 teaspoon chili powder, 1 teaspoon cumin, 1/4 teaspoon salt to beef, stir to combine.
Prepare bell peppers for stuffing: cut the tops off, stem and seed them.
Stuff each bell pepper with the filling. Fill the baking dish with marinara spaghetti sauce and up to 2/3 cups of water (don’t pour all water at once – just pour enough to make the marinara sauce less dense). Place stuffed bell peppers tightly into the prepared baking dish, cavity side up. In my case, I used 2 large bread pans. Top each pepper with both grated Mozzarella and Cheddar cheese. Sprinkle some cheese over the tomato sauce too. Cover with foil
Bake for 30-40 minutes, until the peppers are tender and the filling is hot. Serve in shallow bowls, with some tomato sauce on the bottom.