Brown ground beef in a large skillet over medium-high heat, breaking it up with a spatula. Drain off any excess grease.
Add chopped onion to the skillet and cook until softened, about 5-7 minutes.
Stir in taco seasoning and tomato sauce. Simmer for 5 minutes.
Add black beans and corn to the meat mixture. Stir well to combine.
In a medium bowl, combine softened cream cheese, sour cream, and milk. Beat with an electric mixer until smooth and creamy.
Lightly grease a 9×13 inch baking dish.
Spread a thin layer of the meat mixture on the bottom of the baking dish.
Lay 3-4 tortillas over the meat mixture, overlapping as needed to cover the bottom of the dish.
Spread half of the cream cheese mixture over the tortillas.
Sprinkle 1/3 of the shredded cheddar cheese over the cream cheese mixture.
Repeat layers: Add another layer of meat mixture, followed by a layer of tortillas, the remaining cream cheese mixture, and another 1/3 of the shredded cheddar cheese.
Top with the remaining meat mixture and the remaining tortillas. Again, overlap the tortillas as needed to cover the entire surface.
Sprinkle the remaining shredded cheddar cheese evenly over the top layer of tortillas.
Cover the baking dish with aluminum foil.
Bake in the preheated oven for 20 minutes.
Remove the aluminum foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Let the casserole stand for 10 minutes before cutting and serving.
Serve hot, topped with your favorite taco toppings.
Notes
Spice it up: Add a pinch of cayenne pepper or a dash of hot sauce to the meat mixture for extra heat.
Use different meat: Substitute ground turkey or chicken for the ground beef.
Add vegetables: Include diced bell peppers, zucchini, or other vegetables to the meat mixture.
Make it vegetarian: Use a plant-based ground meat substitute or add more beans and vegetables.
Use different cheese: Experiment with different types of cheese, such as Monterey Jack, pepper jack, or a Mexican cheese blend.
Add a layer of refried beans: Spread a layer of refried beans on the bottom of the baking dish before adding the meat mixture.
Make it ahead: Assemble the casserole ahead of time and store it in the refrigerator until ready to bake. You may need to add a few extra minutes to the baking time if baking from cold.
Freezing Instructions: Assemble the casserole in a freezer-safe dish. Cover tightly with plastic wrap and then aluminum foil. Freeze for up to 3 months. Thaw completely in the refrigerator before baking as directed.