Place cleaned mussels in a saucepan with water, cider, thyme and garlic. Cover and cook over high heat, shaking pan frequently, 6-7 minutes or until shells open. Discard any mussels which remain closed. Cool mussels in liquid. Drain off cooled liquid, strain through a fine sieve and reserve. Discard thyme. Remove a half shell from each mussel and arrange mussels on 4 serving plates.
Melt butter in a saucepan. Add shallots and celery and cook gently 5 minutes. Add Curry Powder and flour and cook 1 minute. Stir in 1 cup of reserved liquid. Bring to a boil, stirring constantly. Cover and cook gently 10 minutes, stirring frequently. Cool. Stir in half-and-half and mayonnaise and mix well. Spoon sauce over mussels and garnish with dill sprigs, if desired. Serve with bread.