For fish stock: Pour water into large pot and add bones, white wine, carrots, onion and leek. Simmer for 30 minutes over low heat. Strain and discard bones and vegetables. Season stock with salt and pepper and reduce to half its volume.
Poach fillets with Vermouth and 3/4-cup stock in large pan over low heat for 10 minutes.
Pour off most of stock from fillets into saucepan and bring to a boil. Meanwhile, sprinkle fish with remaining vermouth and season. Keep warm on low heat.
Strain the boiling stock into a clean pan and reduce over high heat to 1/2 cup. Add cream and boil again. Beat egg yolks in bowl and add cream sauce to yolks. Pour all back into pot and reduce heat to low. Do not boil. To serve, arrange fillets on a platter, coat with sauce, and garnish with sliced truffle.