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