In a large skillet over medium heat, add the chopped onion and minced garlic. Sauté until the onion is translucent, about 3-4 minutes.
Add the ground turkey or beef to the skillet, cooking until browned, breaking it up with a spatula, about 5-7 minutes.
Stir in the diced bell peppers, cooked quinoa or rice, diced tomatoes (with juices), black beans, corn, chili powder, cumin, salt, and pepper. Combine everything thoroughly and cook for an additional 5 minutes, allowing the flavors to meld together.
Transfer the mixture to a greased 9x13-inch casserole dish.
Sprinkle the shredded cheese generously over the top of the casserole.
Cover with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Once out of the oven, let the casserole sit for a few minutes before serving.
Garnish with fresh cilantro if desired.
Notes
Feel free to customize the ingredients based on your preferences.