Preheat the oven to 350°F. Spray a 13x9-inch baking dish with nonstick spray.

Cook the pasta to al dente according to the package directions.

In a large skillet or Dutch oven over medium heat, brown the ground beef and chopped onion. Drain and return to the pan. Add the taco seasoning and water and stir to combine.

Add the cooked pasta and enchilada sauce to the pan and mix well.

Transfer the mixture to the prepared baking dish. Sprinkle with shredded Mexican cheese and top with cubes of American cheese or Velveeta.

Bake until the cheese is melted and just beginning to brown, about 30 minutes.

Remove from the oven and sprinkle with sliced green onions. Serve topped with black olives and a dollop of sour cream if desired.