Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Cook egg noodles according to package directions until al dente. Drain and set aside.
In a large bowl, combine cooked noodles, chicken, cream of chicken soup, cream of mushroom soup, sour cream, milk, peas, celery, onion, 0.5 cup cheddar cheese, garlic powder, salt, and pepper. Mix well.
Transfer mixture to the prepared baking dish and spread evenly.
In a small bowl, mix breadcrumbs, melted butter, and remaining 0.5 cup cheddar cheese. Sprinkle evenly over the casserole.
Bake uncovered for 40–45 minutes, or until the top is golden brown and the casserole is bubbly.
Let cool for 5–10 minutes before serving.
Notes
For a crispier topping, broil for the last 2–3 minutes of baking. Leftovers can be stored in an airtight container in the refrigerator for up to 3 days.