Heat the oven to 400º. Heat the oil in a large skillet over medium heat. Add the onion, garlic and bell peppers and cook them for 3 minutes, stirring often. Add the beef and use a wooden spoon or a spatula to break it up while it cooks, until it is no longer red, about 3 minutes. Stir in 1/2 cup of the salsa, the chili powder (if you like it HOT, feel free to add more), and the salt and pepper. Cook the mixture over low heat, stirring occasionally, for 10 minutes.