Rinse sole in water mixed with the lemon juice.
Pat dry with a cloth.
Sprinkle each piece on one side with salt and pepper, and dot each with about 3/4 teaspoon butter.
Fold each piece over in half lengthwise and place in a shallow baking dish.
Scatter onion and garlic slices over sole.
Sprinkle with peppercorns; tuck in bay leaf.
Add wine and water.
Cover and poach in a moderate oven (350°) for 15 minutes.
Drain and save fish stock; discard onions, garlic, peppercorns, and bay leaf.
Handle folded pieces carefully, for they fall apart easily.
Melt the 3 tablespoons butter in a saucepan; blend in flour, salt, and cayenne.
Slowly add fish stock and cream; stirring constantly, cook until thickened.
Arrange fish on a board or ovenproof platter.
Prepare instant mashed potatoes according to package directions; put through a pastry bag, using rosette tip, to decorate board edge.
Spoon hot sauce over fish, and garnish with large mushroom crowns sauteed in butter; sprinkle with crushed chervil.
Place in a very hot oven (450°) for about 5 to 7 minutes, lightly browning potato border.