Preheat your oven to 350°F (175°C).
In a large skillet, cook the ground beef and chopped onion over medium heat until the beef is browned and the onion is soft. Drain any excess fat.
In a large mixing bowl, combine the cooked beef mixture, diced tomatoes with green chilies, cream of chicken soup, cream of mushroom soup, sour cream, 1 cup of shredded cheddar cheese, chili powder, garlic powder, salt, and pepper. Mix well.
In a 9×13 inch baking dish, spread a layer of slightly crushed Doritos on the bottom.
Top with half of the beef mixture.
Add another layer of crushed Doritos.
Top with the remaining beef mixture.
Finish with a final layer of crushed Doritos and sprinkle the remaining 1 cup of shredded cheddar cheese on top.
Cover the baking dish with foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving.
Garnish with chopped green onions if desired.