In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until it becomes translucent.
Add the lean ground beef to the skillet with the onions. Cook until the beef is browned, breaking it apart with a spatula as it cooks, about 5-7 minutes. Drain any excess fat if necessary.
Sprinkle in the chili powder, garlic powder, and ground cumin. Stir to combine and cook for another minute, allowing the spices to infuse into the beef.
In a large bowl, combine the cooked beef mixture with the drained black beans, drained corn, and salsa. Mix well to ensure all ingredients are evenly distributed.
In your baking dish, spread half of the crushed Doritos on the bottom. Pour the beef mixture over the Doritos, and then sprinkle 1 cup of the shredded Mexican cheese on top. Finish with the remaining Doritos and the rest of the cheese.
Preheat your oven to 350°F (175°C). Place the casserole in the oven and bake for 20-25 minutes, or until the cheese is bubbly and golden brown.
Once baked, remove the casserole from the oven and let it cool for a few minutes. Top with your choice of fresh toppings such as diced tomatoes, chopped cilantro, sliced scallions, diced avocado, black olives, a dollop of sour cream, and shredded lettuce. Serve warm and enjoy!