Preheat oven to 400F.
Spread the tater tots out in a single layer on a large baking sheet. Bake at 400F for 15 minutes.
While the tater tots are baking, heat the olive oil in a large skillet over medium heat. Add the onions and sauté until softened, about 3-4 minutes. Add the garlic and sauté for 1 minute. Do not allow the garlic to brown.
Crumble the ground beef into the skillet and sauté, stirring often, until completely browned. Drain off excess fat. Add the taco seasoning and water to the meat and stir well. Simmer over medium heat for 1-2 minutes, then remove from the heat.
In a large mixing bowl, combine the ground beef mixture, black beans, corn, green chiles, and enchilada sauce. Add 2 cups of the shredded cheese and stir well.
Grease a 9x13 inch glass baking dish with non-stick cooking spray. Pour the enchilada filling mixture into the dish. Arrange the tater tots over the top in a single layer. Top the casserole with the remaining 1 cup of shredded cheese.
Reduce the oven heat to 375F. Bake the casserole uncovered for 30 minutes or until the cheese is melted and the casserole is bubbling around the edges.
Cool the casserole slightly, then serve with the toppings of your choice.