Preheat and prep: Set the oven to 375°F (190°C). Lightly oil a 9x13-inch baking dish.
Warm the tortillas in the microwave for 20–30 seconds so they’re pliable.
Sauté aromatics: Heat olive oil in a skillet over medium. Add onion and bell pepper. Cook 4–5 minutes until softened.
Stir in taco seasoning and cumin; toast 30 seconds until fragrant.
Build the saucy mix: Add salsa and tomato sauce to the skillet. Simmer 2 minutes. Taste and season with salt and pepper.
Remove from heat and stir in Greek yogurt for creaminess.
Combine the filling: In a large bowl, mix shredded chicken, black beans, corn, and brown rice. Pour in about two-thirds of the sauce and toss to coat. The filling should be moist but not soupy.
Layer the casserole: Spread a thin layer of the remaining sauce in the baking dish.
Add a layer of tortillas (tear to fit). Spoon half the filling over tortillas, sprinkle with one-third of the cheese. Repeat layers.
Top with a final tortilla layer, the last of the sauce, and the remaining cheese.
Bake: Cover with foil and bake 15 minutes. Uncover and bake another 10–12 minutes until the cheese is melted and bubbly and the center is hot.
Rest and serve: Let it rest 10 minutes to set slices. Garnish with cilantro, avocado, a squeeze of lime, or jalapeños.
Slice and serve warm.