Preheat the oven to 350°F (175°C).
- In a large skillet, cook the ground beef over medium heat until browned, breaking it apart as it cooks. Drain any excess fat. Add the chopped onion and cook for another 2-3 minutes until softened.
- Stir in the taco seasoning mix and follow the package instructions, simmering for 3-4 minutes.
- Add the cream of chicken soup, sour cream, and diced tomatoes (with juice) to the beef mixture. Stir until well combined and bring to a simmer. Cook for 5 minutes.
- In a greased 9×13-inch baking dish, spread half of the crushed Doritos in an even layer. Pour half of the beef mixture over the Doritos, followed by half of the shredded cheese. Repeat the layers with the remaining Doritos, beef mixture, and cheese.
- Cover with aluminum foil and bake for 20-25 minutes. Remove the foil and bake for another 5-10 minutes, or until the cheese is melted and bubbly.
- Let the casserole cool for a few minutes before serving. Garnish with green onions or cilantro if desired.