Preheat your oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned, about 5-7 minutes. Drain any excess fat.
Stir in the black beans, corn, diced tomatoes, and taco seasoning. Mix well and let it simmer for 5 minutes. Season with salt and pepper to taste.
In a greased 9x13-inch baking dish, spread the beef mixture evenly. Top with sour cream, then sprinkle the shredded cheddar cheese over the top.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for a few minutes. Top with crushed tortilla chips, shredded lettuce, diced tomato, and avocado before serving.