Preheat your oven to 350°F (175°C).
In a large skillet over medium heat, cook the ground beef and diced onion until the beef is browned and the onion is soft, about 5-7 minutes. Drain any excess fat.
Stir in the black beans, corn, diced tomatoes (with juices), and taco seasoning. Cook for an additional 3-4 minutes until heated through.
In a large mixing bowl, combine the crushed Doritos, 1 cup of cheddar cheese, sour cream, and milk. Mix until well combined.
In a greased 9x13-inch baking dish, layer half of the Dorito mixture on the bottom. Top with the beef mixture, then add the remaining Dorito mixture on top.
Sprinkle the remaining 1 cup of cheddar cheese over the top.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for 5 minutes before serving. Garnish with chopped cilantro if desired.