Preheat the oven to 350 degrees F.
Spray a 13x9 baking dish with cooking spray. Set aside.
In a large skillet, brown the ground beef until almost done.
Add the onions and cook until they turn soft and translucent, about 3-4 minutes and the beef is no longer pink.
Stir in the taco seasoning and water, and heat to boiling.
Reduce the heat and stir in the beans, Rotel tomatoes, and sour cream.
Cook for 8-10 minutes or until heated through and the mixture is slightly thickened.
Add a single layer of corn chips to the bottom of the prepared baking dish.
Sprinkle 1 cup of the shredded cheese.
Top with the beef mixture.
Sprinkle with the remaining cheese.
Cover with foil and bake for 15 minutes, then remove the foil and continue baking for 5 minutes or until the cheese is melted and the casserole is bubbly around the edges.
Remove from the oven and top with the remaining corn chips and toppings of your choice.
Serve immediately.
Store the leftovers in the refrigerator.