Preheat your oven to 350°F (175°C). Lightly grease a 9×13 inch baking dish with cooking spray or oil. This step prevents sticking and helps the casserole cook evenly.

Heat olive oil in a small pan over medium heat. Add chopped onion and cook for about 3 minutes. Stir often until the onion becomes soft and slightly golden.

In a large mixing bowl, combine cream of chicken soup, sour cream and milk. Add garlic powder and black pepper. Stir well until the mixture becomes smooth and creamy.

Add shredded chicken to the creamy mixture. Stir well so every piece of chicken is coated with the sauce. This creates the rich base of the casserole.

Mix in the cooked onions and diced tomatoes with green chilies. Stir gently to combine everything. These ingredients add extra flavor and a little freshness.

Spread half of the crushed Doritos on the bottom of the baking dish. This crunchy layer gives the casserole a fun texture and strong cheesy flavor.

Pour the creamy chicken mixture over the Doritos layer. Spread it evenly with a spoon so the casserole cooks evenly and each bite has balanced flavor.

Sprinkle cheddar cheese and mozzarella cheese evenly over the casserole. Then add the remaining crushed Doritos on top. This layer becomes golden and crispy.

Place the baking dish in the oven. Bake for about 30 minutes until the cheese melts and bubbles. The top should look golden and slightly crispy.

Remove the casserole from the oven. Let it rest for about 5 minutes before serving. This helps the layers set and makes serving easier.