Preheat oven to 350 degrees F. Spray a 13x9 inch baking dish with cooking spray and set aside.

In a large skillet over medium heat, brown the ground beef for 5 to 6 minutes breaking it apart as it cooks. Add finely chopped onion and cook for 3 to 4 more minutes until softened and beef is completely cooked through with no pink remaining. Drain all excess grease from the skillet.

Stir in taco seasoning and water. Bring to a boil then reduce heat to low. Stir in drained black beans, Rotel tomatoes, and sour cream until fully combined. Simmer for 8 to 10 minutes stirring occasionally until the filling is thickened and creamy and holds together on a spoon.

Spread a single even layer of corn chips across the entire bottom of the prepared baking dish. Sprinkle 1 cup of shredded cheese evenly over the chips. Spoon the beef filling over the cheese and spread it to the edges. Sprinkle remaining 1 cup of shredded cheese evenly over the top.

Cover tightly with aluminum foil and bake for 15 minutes. Remove foil and bake for another 5 minutes until cheese is fully melted and casserole is bubbling around the edges.

Remove from oven and immediately scatter fresh corn chips over the top. Add desired toppings such as sour cream, guacamole, sliced green onions, salsa, and jalapeños. Serve immediately.