Rinse the crawfish after peeling under cold water. Place the crawfish in a pan. Pour over 2 cups of white wine, just enough water to cover the crawfish. Add the onion cut by half, 2 whole carrots, 1 lemon cut in half, dill stems, leek, salt peppercorn and cayenne pepper. Bring the liquid slowly to a boil. Simmer 3 minutes. Turn the heat off and leave covered in the liquid until putting the crawfish in the Beurre Blanc.
Beurre Blanc Sauce: Place in a saucepan the shallots with a 1/2 cup of white wine, lemon juice (2 lemons). Bring to a boil, uncovered, until the liquid is reduced to about half. Strain the liquid into a small saucepan over very low heat. Whisk the butter (1 1/4 cups) a piece at a time, making sure each piece is thoroughly incorporated before adding the cream. When all the butter has been added, whisk in the chopped chervil, tarragon, paprika, salt, pepper and cayenne pepper.
Remove the crawfish from the pan and out in the Beurre Blanc.
Place 2 cups of butter in a saute pan, melt the butter on medium heat. Put salt and pepper on the fish filet, dust in the flour and place the filets in the pan for 3 to 4 minutes on one side until golden brown, turn over for another 3 to 4 minutes. Put in the plate to serve, and accompany with the crawfish Beurre Blanc sauce and seasonal vegetables.