Step 1In a small, heavy-bottomed saucepan, gently heat one-half cup water, the butter and the salt over medium-low heat until the butter is melted.
Step 2Bring the butter mixture just to a boil (prolonged boiling evaporates the water and changes the proportions of the dough). Remove the saucepan from the heat and immediately add all the flour. Beat vigorously with a wooden spoon for a few moments until the mixture pulls away from the sides of the pan to form a ball. Beat for about 3 minutes more over low heat to dry the dough. Take the pan off the heat.
Step 3In a small bowl, beat one egg until mixed; set it aside. In a separate small bowl, beat the second egg; mix it directly into the dough. Then beat in enough of the reserved egg (about 1 tablespoon) so the dough is shiny and just falls from the spoon. Beat the egg quickly into the dough -- it needs to incorporate into the heated dough so it does not scramble. Cover the dough in the pan lightly with plastic wrap to prevent a skin forming, and set it aside until cool.
Step 1Wash and dry the fish fillets and cut them into about 1-inch pieces. Puree them in a food processor until smooth, continuing until the fish forms a ball that pulls away from the sides of the bowl. Using a bench scraper or spatula, push the fish puree through a tamis or fine mesh strainer to remove any bone fragments and sinew.
Step 2Place the fish puree back in the processor, and with the blades running, gradually add the egg whites, taking 1 to 2 minutes. Transfer the mixture to a metal bowl and set it over ice. Chill until very cold, stirring occasionally, 15 to 20 minutes. Return the mixture to the food processor and beat in the cooled, choux pastry dough using the pulse button. Gradually beat in 1 cup creme fraiche or cream, followed by 1 teaspoon salt, one-fourth teaspoon pepper and one-eighth teaspoon grated nutmeg, or to taste. If the quenelle mixture is soft, you'll find the salt will stiffen it slightly. Return the mixture to the metal bowl and chill again over ice until very cold.
Step 3To poach the dumplings, fill a shallow stockpot with a 3-inch layer of water, add about 2 teaspoons salt, and bring to a simmer. Holding a large tablespoon in each hand, dip each spoon in the pan of hot water so the quenelle mixture does not stick, then take a spoonful of mixture and shape, forming an oval. Drop the dumpling into the simmering water and simmer for 2 to 3 minutes. If this test dumpling starts to break up, add another lightly beaten egg white to the remaining mixture and beat for 2 to 3 minutes over ice. If the dumpling holds together, continue simmering for a total of 10 minutes. Lift out and taste the cooked quenelle, and adjust the seasoning of the mixture if desired. Shape more ovals and drop them into the simmering water. Poach for about 10 minutes depending on size, or until they are firm when you press them with a fingertip. Lift them out with a slotted spoon and drain on paper towels. This makes about 20 quenelles.
Step 1Melt one-third of the butter in a large shallow pan and saute the onion and carrot about 2 minutes over medium heat. Add the crayfish, cover and saute over high heat, stirring often, until they turn red, 3 to 4 minutes. Add the Cognac off the heat, then carefully tilt the pan over the flame to ignite the Cognac and flambe the crayfish until the flames die out.
Step 2Add the white wine, fish stock, bouquet garni, salt and pepper -- the crayfish should be partly submerged, and if necessary add more stock. Cover the pan and simmer the crayfish 8 more minutes to fully cook (10 minutes if the crayfish are very large).
Step 3Remove the crayfish from the pan until cool enough to handle, about 10 minutes. Pull the heads from the tails, and shell the tails as with shrimp. Make sure to remove the intestinal vein (this runs lengthwise along the top of the tail). If it does not come off with the shell, it should pull off easily. Set the tail meat aside. Put the shells in a heavy-duty plastic bag and pound with a rolling pin to crush them. Return the shells to the cooking liquid over high heat until it begins to simmer, then reduce the heat and simmer 10 to 12 minutes to allow the shells to steep in the liquid. Strain the liquid into a bowl, pressing hard to extract maximum flavor from the shells, and set the liquid aside.
Step 4In a medium heavy-bottomed saucepan, melt a second portion of butter over medium-low heat, whisk in the flour and cook until it foams, about 1 minute. Whisk in the crayfish liquid and bring to a boil, whisking constantly until the sauce thickens. Whisk in the creme fraiche or cream, bring just back to a boil and, if you like, add tomato paste to deepen the color of the sauce. Season the sauce with one-half teaspoon salt, one-eighth teaspoon pepper, a pinch of cayenne pepper and a trickle of Cognac, or to taste. Remove the sauce from the heat and whisk in the remaining butter, cut into one-half inch pieces.
Step 5Heat the oven to 375 degrees. Arrange three quenelles, each in buttered, individual (6-inch) baking dishes, or all in one large (9-inch by 13-inch) baking dish, and scatter the reserved crayfish tails around them. Coat the quenelles generously so they bathe in the sauce. They can be prepared up to this step a day ahead and stored covered in the refrigerator.
Step 6Bake the quenelles until bubbling and puffed, 10 to 15 minutes for individual dishes, 20 to 25 minutes for a large dish. Serve very hot in the baking dishes.