01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or non-stick spray.
02 - In a large mixing bowl, combine the diced chicken, chopped onion, thawed peas and carrots, 1 1/2 cups of 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 arranged tater tots to promote browning during baking.
06 - Bake uncovered at 375°F for 40 minutes until the tater tots begin to crisp and the filling bubbles at the edges.
07 - Remove from the oven and sprinkle the remaining 1/2 cup of cheddar cheese 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 brown and crispy.
08 - Allow the casserole to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired, then serve warm.