1. Drain the oysters through a fine sieve set over a bowl, reserving all their liquor. Pat the oysters completely dry with paper towels, cut each of them into thirds and set aside. Measure the liquor and add enough fish stock or clam broth to make 2 cups.
2. In a heavy 2- to 3-quart saucepan, melt the butter over moderate heat. When the foam begins to subside, add the celery and scallions and, stirring frequently, cook for about 5 minutes, or until the vegetables are soft but not brown. Add the flour and mix well.
3. Stirring constantly with a wire whisk, pour in the reserved 2 cups of oyster liquor in a slow, thin stream and cook over high heat until the sauce comes to a boil, thickens lightly and is smooth.
4. Reduce the heat to low, add the oysters, salt and red pepper, and simmer uncovered for 2 to 3 minutes, or until the oysters plump up. In a small bowl, beat the egg yolks with a fork or wire whisk.
5. When they are smooth, ladle in about 2 tablespoons of the simmering liquid and mix well. Then, stirring constantly, pour the heated egg yolks gradually into the soup.
6. Still stirring, cook over low heat for 2 to 3 minutes longer, until the soup thickens heavily. Do not let the soup come anywhere near a boil or the egg yolks will curdle.
7. Taste for seasoning, stir in the parsley and serve the oyster soup at once from a heated tureen or individual soup plates.
