Preheat the Oven: Preheat your oven to 350°F (175°C). Lightly grease a 2-quart casserole dish with nonstick cooking spray.
Mix the Filling: In a large mixing bowl, combine the shredded chicken or ground beef, 1 cup of shredded cheese, cream of chicken soup, Rotel tomatoes (including the juice), sour cream, milk, and taco seasoning. Stir until evenly blended.
Layer the Casserole: Spread half of the crushed Doritos on the bottom of the casserole dish. Top with half of the meat mixture, spreading it evenly. Repeat the layers with the remaining Doritos and meat mixture.
Add Cheese: Sprinkle the remaining shredded cheese over the top. Add extra cheese if you prefer a richer, gooier casserole.
Bake: Cover the casserole dish with aluminum foil and bake for 35 minutes. If doubling the recipe for a larger dish, increase the baking time to 45 minutes.
Rest and Serve: Remove the casserole from the oven and let it rest for 5-10 minutes to set. Serve warm with your favorite sides or garnishes.