Melt butter in skillet. Add shallots, wine, and clam juice. Boil 3 minutes. Take from heat. Slash skinned side of fillets several times to prevent warping. Lay fillets, skinned side down, in hot liquid. Butter a piece of foil and put directly on the fillets. Set a lid down inside the pan on top of the foil. This keeps the foil and fillets down in the liquid. Bring to a boil, then reduce to a simmer. Poach 5 minutes.
While fillets poach, blend together softened butter and flour. This is beurre manié. (Many times in this book it has been suggested to keep this mixture on hand for use in thickening. If you have some, use it; if not, make a double batch now and freeze what you don’t use.) Stir together the yolk and crème ready to use.
When fillets are done, put on hot serving platter. Add ¼cup whipping cream, salt, pepper and sugar to skillet, heat, then stir in sufficient beurre manié to thicken. Add 1tablespoon, stir into the sauce, and then add more to thicken. Make the sauce a little too thick to allow for the egg-crème mixture. Remove from heat, stir in liaison of yolk and crème and carefully heat, but do not boil. Taste and adjust seasonings, then pour over fillets. Decorate platter with parsley and lemon halves.