Preheat the oven to 350°F (175°C). Lightly grease a 13×9-inch baking dish with nonstick cooking spray.
In a large skillet over medium heat, cook the ground beef until nearly browned. Add the chopped onion and continue cooking until the onion is tender and the beef is fully cooked.
Stir in the taco seasoning and water. Bring the mixture to a gentle boil.
Reduce the heat and mix in the black beans, diced tomatoes with green chiles, and sour cream. Simmer for 8–10 minutes, stirring occasionally, until slightly thickened.
Spread a single layer of Fritos corn chips across the bottom of the prepared baking dish.
Sprinkle 1 cup of shredded Mexican cheese over the chips.
Evenly spread the beef mixture over the cheese layer.
Top with the remaining 1 cup of shredded cheese.
Cover with foil and bake for 15 minutes.
Remove the foil and continue baking for 5 minutes, or until the cheese is melted and the casserole is bubbling around the edges.
Remove from the oven and let rest for 5 minutes.
Top with additional Fritos and your favorite toppings before serving.