Preheat the oven: Set your oven to 375°F (190°C).
Lightly grease a 9x13-inch baking dish.
Cook the aromatics: Heat olive oil in a large skillet over medium heat. Add onion and bell pepper. Sauté 4–5 minutes until softened, then stir in garlic for 30 seconds.
Brown the meat: Add the ground turkey or beef to the skillet.
Cook, breaking it up with a spatula, until no longer pink. Drain excess fat if needed.
Season and simmer: Sprinkle in taco seasoning, then add broth. Stir and simmer 2 minutes to hydrate the spices and create a saucy base.
Add the bulk: Stir in drained tomatoes, black beans, and corn (if using).
Fold in the cooked quinoa or rice. Taste, then add salt and pepper if needed.
Make the creamy layer: In a small bowl, mix Greek yogurt with salsa until smooth. This takes the place of sour cream while adding extra protein.
Assemble the casserole: Spread the meat-and-bean mixture evenly in the prepared baking dish.
Spoon the yogurt-salsa mixture over the top and spread gently. Sprinkle 1 cup shredded cheese evenly over everything.
Bake: Place in the oven and bake for 18–22 minutes, until the cheese is melted and the edges are bubbling.
Finish and serve: Remove from the oven and sprinkle with remaining 1/2 cup cheese. Let it rest for 5 minutes so it sets.
Top with chopped cilantro and any optional toppings. Slice into squares and serve warm.