Preheat your oven to 375 degrees Fahrenheit. Lightly grease a 9×13 inch baking dish.
Cook the ground beef in a skillet over medium heat until browned. Drain off any excess fat. If using shredded chicken, skip this step.
Stir the chopped onion into the skillet with the beef (if using beef) and cook until softened, about 3 minutes.
In a large bowl, mix together the cream of chicken soup, sour cream, Rotel tomatoes (with liquid), chili powder, and cumin.
If using ground beef, add the cooked beef and onion mixture to the soup mixture. If using shredded chicken, add the chicken to the soup mixture. Stir well to combine.
Fold in the cheddar cheese and Monterey Jack cheese, reserving about 1/4 cup of the cheese blend for topping.
Spread half of the mixture into the prepared baking dish. Sprinkle half of the crushed Doritos over the layer.
Spread the remaining mixture over the chips. Top with the remaining cheese blend and the rest of the crushed Doritos.
Bake for 20 to 25 minutes, or until the casserole is bubbly and the topping is golden brown.
Let the casserole rest for 5 minutes before serving.
Notes
For a freezer meal, assemble the casserole completely (do not bake), cover tightly with foil, and freeze for up to 3 months. Thaw overnight in the refrigerator before baking, adding 5-10 minutes to the bake time if baking from cold.
You can substitute cream of mushroom soup for cream of chicken soup if desired.
Use any flavor of Doritos you prefer, such as Cool Ranch, for a different taste profile.