Prep all ingredients by chopping the onion and bell pepper, mincing the garlic, and gathering canned ingredients.
In a large skillet, brown the ground beef over medium heat for 5 to 7 minutes. Drain excess fat, then add onion, bell pepper, and garlic. Sauté until tender, about 5 minutes.
Stir in diced tomatoes, kidney beans, tomato sauce, chili powder, cumin, salt, and pepper. Let it simmer for 5 minutes, then transfer to a baking dish. If using, add cooked rice and frozen mixed vegetables.
Sprinkle shredded cheddar cheese on top, cover with foil, and bake at 350°F for 30 minutes. Remove foil and bake for an additional 10 to 15 minutes until cheese is bubbly and golden.