1. Bring the court bouillon to a boil. Add the shrimp, cover and cook three minutes. Remove the shrimp with tongs, leaving the court bouillon in the pan.
2. Add the mussels, cover and steam gently three to four minutes, or until they open.
3. Shell and devein the shrimp, and cut in half lengthwise. Remove the mussels from their shells and keep the mussels and the shrimp warm.
4. Gently boil the court bouillon left in the pan while preparing the fish.
5. In a large skillet melt three tablespoons of the butter. Brush the fillets with the butter and sprinkle with salt and pepper. Place the shallot in the bottom of the skillet and arrange the fillets and mushrooms over the top. Add the wine and water and cover with a circle of buttered waxed paper the size of the skillet.
6. Bring the wine mixture to a boil and poach the fillets gently until the fish flakes easily when tested with a fork, four to five minutes. Using two wide spatulas, remove the fillets and mushrooms to a heated broilerproof buttered
serving platter and keep warm.
7. Strain the liquid left in the pan and strain the boiling court bouillon.
8. Melt the remaining butter in the same skillet, add the flour and stir with a wire whisk until blended. Meanwhile, bring one and one-half cups of the strained liquid with the light cream to a boil and add all at once to the butter-flour mixture, stirring vigorously with the whisk until the sauce is thickened and smooth.
9. Add the egg yolk lightly beaten with a little of the hot sauce and cook, stirring, one minute. Correct the seasonings.
10. Arrange the mussels and shrimp over the fillets and pour the sauce over the top. Heat very quickly under a preheated broiler and garnish with parsley or sprigs of thyme.