Preheat your oven to 350°F (175°C). Gather your baking dish and cooking utensils.

In a large skillet over medium heat, cook the ground beef until browned, about 5–7 minutes. Add the chopped onion and cook until translucent, about 3–4 minutes. Stir in the taco seasoning and water, simmering for 2–3 minutes, then mix in the black beans, diced tomatoes with green chiles, and sour cream until well-combined.

Layer Fritos in the bottom of the baking dish, followed by half of the shredded cheese. Top with the meat mixture, then add the remaining cheese on top.

Cover with foil and bake for 15 minutes. Remove the foil and bake for another 5 minutes or until cheese is bubbly and golden.

Let casserole cool slightly and serve with Fritos and desired toppings like green onions, sour cream, guacamole, salsa, and jalapenos.