Preheat oven to 350 degrees F
Spray 13x9 baking dish with cooking spray
In a large skillet, brown ground beef until almost done
Add onions and cook until soft, about 3-4 minutes, and beef is no longer pink
Stir in taco seasoning and water, heat to boiling
Reduce heat and stir in beans and enchilada sauce
Cook 2-3 minutes or until heated through and mixture is slightly thickened
Add about 3 ½ cups of corn chips to the bottom of the prepared baking dish
Sprinkle with half the shredded cheese (about 1 cup)
Top with beef mixture
Sprinkle on the remaining cheese
Bake uncovered for 18-20 minutes or until cheese is melted and casserole is bubbly around the edges
Remove from oven and top with remaining corn chips and toppings of your choice, if desired
Serve immediately
Store leftovers in refrigerator