Preheat oven to 375°F (190°C). Brown ground turkey or beef with onion in a skillet over medium heat. Drain any excess fat.
Stir in cooked rice, black beans, corn, diced tomatoes, salsa, taco seasoning, and chili powder. Cook until heated through.
Transfer mixture to a greased casserole dish and spread evenly.
Top with shredded cheddar cheese, spreading evenly across the dish.
Bake for 15–20 minutes or until cheese is melted and bubbly.
Garnish with fresh cilantro. Serve warm with tortilla chips, sour cream, or guacamole.