1. If using the eggs, separate the yolks from the whites and save the whites for another use. Press the yolks through a sieve and reserve.
2. In a large pot, melt the butter over moderately low heat. Add the scallion and celery and cook, stirring occasionally, until the vegetables are soft, about 4 minutes. Add the flour and cook, stirring, for 2 minutes. Stir in the stock, milk and cream. Bring to a simmer, stirring.
3. Add the sieved egg yolks, if using, the crabmeat, sherry, salt and pepper and bring almost to a simmer. Ladle into bowls and top each serving with a pinch of paprika and a lemon slice.