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 translucent, about 5-7 minutes. Drain any excess fat.
Stir in the taco seasoning, black beans, corn, and diced tomatoes. Cook for an additional 5 minutes, allowing the flavors to meld.
In a greased 9x13-inch baking dish, layer half of the Fritos corn chips on the bottom.
Spread the beef mixture evenly over the Fritos layer.
Top with half of the shredded cheddar cheese and dollops of sour cream.
Add the remaining Fritos on top, followed by the rest of the cheese.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for 5 minutes before serving.
Garnish with sliced green onions, shredded lettuce, and diced tomatoes.