Mix cream of celery soup, milk, curry powder, and mustard powder in a large bowl. Stir rice, tuna, and onion into the soup mixture; transfer to a casserole dish.
Mix bread crumbs, melted butter, and parsley in a bowl; sprinkle over tuna mixture.
Bake casserole in preheated oven until bread crumb mixture is golden brown, about 25 minutes; top with sliced eggs and return to over to bake another 5 minutes. Let casserole cool 5 minutes before serving.