01 - Preheat oven to 375°F and grease a 9x13-inch baking dish.
02 - Cook egg noodles according to package instructions until al dente, then drain and set aside.
03 - Melt 1 tablespoon butter in a skillet over medium heat. Sauté onion, celery, and garlic until softened, about 4 minutes.
04 - In a large bowl, mix cooked chicken, noodles, sautéed vegetables, thawed peas and carrots, cream of chicken soup, sour cream, milk, cheddar cheese, thyme, salt, and pepper until well combined.
05 - Spread the mixture evenly in the prepared baking dish.
06 - Combine crushed crackers with melted butter and sprinkle evenly over the casserole.
07 - Bake uncovered for 30 to 35 minutes until the topping is golden and bubbling.
08 - Allow casserole to rest for 5 minutes before serving.