01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with nonstick spray or butter.
02 - In a large mixing bowl, combine the cooked chicken, chopped onion, thawed peas and carrots, 1 1/2 cups of the shredded cheddar cheese, sour cream, condensed cream of chicken soup, garlic powder, salt, and black pepper. Stir until all ingredients are evenly incorporated.
03 - Transfer the chicken mixture into the prepared baking dish and spread it into an even layer across the bottom.
04 - Place the frozen tater tots in a single even layer over the entire surface of the chicken filling.
05 - Pour the melted butter evenly over the tater tots to help them crisp during baking.
06 - Bake uncovered for 40 minutes until the tater tots are beginning to turn golden.
07 - Remove from the oven and sprinkle the remaining 1/2 cup of cheddar cheese over the tater tots. Return to the oven and bake for an additional 5 minutes, or until the cheese is fully melted and the tater tots are golden brown and crispy.
08 - Allow the casserole to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired, and serve warm.