Preheat your oven to 350°F (175°C).
In a large skillet over medium heat, add a splash of oil and sauté the chopped onions and bell pepper until softened (about 5-7 minutes).
Stir in the cooked beans, diced tomatoes, chili powder, cumin, garlic powder, salt, and pepper. Mix well to combine all the ingredients.
Transfer the mixture to a 9x13 baking dish and smooth out evenly.
Sprinkle the shredded cheese on top of the bean mixture.
Bake in the preheated oven for about 30-40 minutes or until the cheese is bubbly and golden brown.
Remove from the oven and let it sit for a few minutes before serving. Enjoy warm!