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 cheddar cheese, sour cream, cream of chicken soup, garlic powder, salt, and black pepper. Stir until evenly incorporated.
03 - Transfer the chicken mixture into the prepared baking dish and spread it into an even layer.
04 - Place the frozen tater tots in a single, even layer across the entire surface of the chicken filling.
05 - Pour the melted butter evenly over the arranged tater tots.
06 - Bake uncovered for 40 minutes until the tater tots begin to crisp and turn golden.
07 - Remove from the oven and sprinkle the remaining 1/2 cup of cheddar cheese evenly over the top. Return to the oven and bake for an additional 5 minutes, or until the cheese is fully melted and the tater tots are golden and crispy.
08 - Allow the casserole to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired, and serve warm.