In a large mixing bowl, combine all the ingredients: cooked chicken, cooked rice, cream of celery soup, chopped onion, drained green beans, mayonnaise, water chestnuts, and sliced pimientos (if using).
Mix well until everything is evenly incorporated.
Pour the mixture into a 2 1/2 or 3 quart casserole dish, spreading it out evenly.
Bake in the preheated oven for 25 to 30 minutes, or until the casserole is heated through and bubbly.
Notes / Tips / Wine Advice:
Wine Pairing:
Pair this comforting Chicken Spectacular with a light-bodied Chardonnay or a crisp Sauvignon Blanc.