Preheat the oven to 375°F (190°C).
In a small bowl, mix together the ingredients for the taco seasoning.
In a large oven-safe skillet over medium heat, cook the ground beef and onions until the beef is browned (about 10 minutes). Remove any excess fat.
Add the taco seasoning mix, water, and RO-TEL to the skillet and sprinkle the cheese on top. Bake for 10 minutes or until the cheese is melted and bubbly.
While the casserole is baking, prepare the toppings.
Once the casserole is done, add the corn chips on top of the melted cheese and garnish with the lettuce, tomatoes, green onions, jalapeno, sour cream, and salsa.