Preheat oven to 350°F. Lightly coat a 7-by-11-inch baking dish with cooking spray.
Bring a medium pot of water to a boil. Cook noodles for 2 minutes less than package directions. Drain and transfer to a bowl.
Heat butter in a large nonstick skillet over medium heat. Add celery, onion, garlic and salt; cook, stirring often, until tender and translucent, about 8 minutes. Stir in flour; cook, stirring constantly, for about 30 seconds. Add broth and half-and-half; cook, stirring constantly, until the liquid thickens, about 4 minutes. Stir in lemon juice, pepper, onion powder and garlic powder.
Remove from heat; stir in chicken, the noodles, peas and carrots and 2 tablespoons dill. Transfer to the prepared baking dish. Cover with foil and bake until hot throughout, about 30 minutes. Sprinkle with the remaining 2 tablespoons dill.
