Preheat the oven: Set oven to 375°F (190°C).
Cook the meat: In a large skillet over medium heat, brown the ground beef (or turkey) with the diced onion, breaking it up as it cooks, about 5–7 minutes. Drain excess fat.
Season the meat: Add taco seasoning, salsa, and diced tomatoes with green chilies. Stir well and simmer for 3–4 minutes until thickened.
Add beans and corn: Stir in black beans and corn, mixing until combined. Remove from heat.
Assemble the casserole: In a greased 9x13-inch baking dish, layer half the crushed tortilla chips. Spread the meat mixture evenly over the chips, then top with 1 cup of shredded cheese. Add another layer of tortilla chips and the remaining cheese.
Bake the casserole: Bake for 20–25 minutes until cheese is melted and bubbly.
Add fresh toppings: Spread sour cream over the top (optional) and garnish with shredded lettuce, avocado, cilantro, or other toppings. Serve warm!