Chef Bagnard was a wonder with sauces, and one of my favorite lessons was his sole a la normande. Put a half-pound of sole fillets in a buttered pan, place the fish's bones on top, sprinkle with salt and pepper and minced shallots. Fill the pan with liquid just covering the fillets, half white wine, half water, plus mussel and oyster juices. Poach. When the fillets are done, keep them warm while you make a roux of butter and flour. Add half of the cooking juices and heat. Take the remaining cooking juices and reduce to almost nothing, about a third of a cup. Add the reduced liquid to the roux and stir over heat. Then comes Bagnard's touch of genius: remove the pan from the fire and stir in one cup of cream and three egg yolks; then work in a mere three-quarters of a pound of butter. I had never heard of stirring egg yolks into such a common sauce, but what a rich difference they made.
Oh, crise de foie, that French sole was delicious!
From My Life in France, by Julia Child with Alex Prud'homme