Prepare an ice bath. Bring a pot of salted water to a boil over high heat. Add the watercress in batches and blanch until wilted but still bright green, about 15 seconds. Refresh the watercress in the ice bath, then squeeze out the excess water.

In a food processor process the watercress and 3 egg whites until almost smooth. Add the heavy cream and continue to process. Place in a strainer set over a bowl. Puree the scallops in the food processor. Add the remaining egg white, salt and white pepper and continue to process. Add the drained watercress mixture to the scallops and process until well mixed. Bring a small pot of water to a boil. Add a small amount of the scallop mixture and poach until cooked through. Taste and adjust seasoning to the mixture, as necessary.

Preheat the oven to 400 degrees F. Lightly grease a large baking sheet. On a lightly floured surface, roll out 1 piece of the puff pastry into a long rectangle, about 20-inches by 8-inches, or large enough for the fillet. Place the puff pastry sheet on the prepared baking sheet. Season both sides of the salmon with salt and white pepper, then place in the middle of the pastry, skin side down.

Spread the watercress mousse evenly over the top of the salmon. Cut the pastry around the salmon to within 2 inches of the fish, leaving a triangular "tail" shape at the bottom and a slightly pointed shape at the "head" end. Brush the edges of the pastry with egg wash. Roll out the remaining pastry sheet on the floured surface and trim to the same size as the first sheet.

Top the salmon with the second pastry sheet and seal the edges to within 3/4-inch of the fish. With a pastry tip, lightly score the top pastry sheet to resemble fish scales, and using the excess pastry, form an eye, mouth and fins. Brush the pastry with the remaining egg wash and bake until golden brown, 20 to 25 minutes. While the fish is baking, make the sauce.

Champagne Chive Butter Sauce: In a medium saucepan combine the shallots and Champagne and bring to a boil. Reduce the heat to medium-low and simmer until reduced by half, about 15 minutes. Add the cream, bring to a boil, and reduce by half, about 3 minutes. Reduce the heat and whisk in the butter, 1 tablespoon at a time, adding each piece before the previous one has been completely incorporated. Continue until all the butter is added and the sauce coats the back of a spoon, removing the pan from the heat periodically to prevent the sauce from getting too hot and breaking. Strain, if desired. Season, to taste, with salt and pepper and fold in the chives.

Remove the fish from the oven and transfer to a large platter or tray decorated with watercress sprigs and present tableside. Cut into individual portions and serve with the Champagne-Chive Butter and new potatoes on the side.

Put the potatoes and 1 teaspoon of salt in a medium, heavy stockpot. Cover with water by 1-inch and bring to a boil. Cook until the potatoes are tender, 12 to 15 minutes. Drain in a colander and return to the pot. Add the remaining ingredients, cover with the lid, and shake to blend. Remove from the heat and adjust the seasoning, to taste. Cover to keep warm until ready to serve

4 to 6 servings