Soak beans over night; in the morning drain and add cold water. Slice onion, and cook five minutes with half the butter, adding to beans, with celery stalks broken in pieces. Simmer three or four hours, or until beans are soft; add more water as water boils away. Rub through a sieve, reheat to the boiling point, and add salt, pepper, mustard, and cayenne well mixed. Bind with remaining butter and flour cooked together. Cut eggs in thin slices, and lemon in thin slices, removing seeds. Put in tureen, and strain the soup over them.