Chicken-Noodle Casserole

4 stalks celery, finely chopped
1 medium onion, finely chopped
1 Tbsp. vegetable oil
2 lb. chicken legs/thighs
1/2 tsp. black pepper
1 tsp. dried thyme, crushed
1/2 tsp. salt
1 slice bread
12 oz. jumbo or extra-large egg noodles
8 oz. container cheddar French onion or French onion flavor sour cream dip
2 Tbsp. all-purpose flour
nonstick cooking spray
2 Tbsp. fresh parsley

Preheat oven to 375. In a Dutch oven, cook 2/3 of the celery and 2/3 of the onion in hot oil over medium heat for 3 min. Add chicken, pepper, thyme, and salt; cook for 2 min. Add 6 C. water. Bring to a boil; reduce heat and simmer, covered, 20-25 min., until chicken is no longer pink.
Meanwhile, for topping, tear bread into small pieces. In a small bowl, toss together bread, remaining celery and onion; set aside.
With a slotted spoon transfer chicken to cutting board. Add noodles to simmering broth; boil gently 7-8 min., just until tender, stirring occasionally. With slotted spoon, transfer noodles, celery, and onions to a 3 qt baking dish that has been sprayed with nonstick cooking spray.
For sauce, in a bowl, whisk together sour cream dip and flour. Gradually whisk in 1 C. of the hot broth until smooth. Add to broth in Dutch oven; cook and stir until boiling.
Meanwhile, discard skin and bones from chicken. Chop chicken, then add to noodles in dish. Gently stir in sauce. Sprinkle with topping, then lightly spray with nonstick cooking spray.
Bake uncovered 30-35 min. until heated through and topping begins to brown. Top with parsley just before serving.