Preheat your oven to 350°F (175°C).

In a large skillet over medium heat, brown the ground beef for about 6-8 minutes. Then, add the finely chopped onion and cook until soft, about 3-4 minutes.

Stir in the taco seasoning and water, mix thoroughly. Increase heat to bring to a gentle boil, then reduce to low and simmer for 2 minutes.

Add the black beans, diced tomatoes with green chiles, and sour cream. Cook over low heat for an additional 3-4 minutes until warmed through.

In a large baking dish, layer Fritos corn chips at the bottom, then half of the beef mixture and a layer of shredded cheese.

Repeat the layering process with the remaining beef mixture and top with the last of the shredded cheese.

Cover with aluminum foil and bake for 15 minutes. Remove foil and bake for an additional 5 minutes, until cheese is bubbly and golden.

Let the casserole cool for a few minutes before serving. Top with extra Fritos and your choice of toppings like green onions, guacamole, salsa, or jalapenos.