To make the mushroom-shrimp stuffing: Heat the tablespoon of butter in a skillet. When it is hot but not smoking, add the shallots. Saute for about 1 minute, until the shallots have softened, then add the mushrooms, and cook over medium to high heat until they have released their juice. When that juice has evaporated and the mixture starts sizzling again, add the shrimp, and cook for about 1 minute, just long enough for the shrimp pieces to change color. Season with the pepper, salt, and chives. Cool.
When ready to assemble the dish, line a large cookie sheet with parchment paper. Roll the dough into a 16-inch square. Cut a 16-inch-long strip from the dough measuring 5 to 6-inches wide, and place it on the diagonal on the parchment paper. Place one of the salmon fillets on top, and sprinkle it lightly with salt and pepper. Spread the cooled mushroom-shrimp stuffing evenly on top, then cover the stuffing with the remaining salmon fillet.
Take care to arrange the fillets so that the thinner part of the fillet on the bottom corresponds to the larger, thicker part of the fillet on top (tail to head and head to tail), and the shape and thickness are the same throughout. This way the salmon will cook evenly.
Roll the remaining, wider strip of dough up onto your rolling pin, then unroll it on top of the second fillet. Brush off any flour from the surface of the dough, and press it all around the edges, so it conforms to the fish shape underneath. The large top layer of dough will stick around the edge to the layer of dough underneath. Press the two layers of dough gently around the edge so they adhere well together, then place in the freezer for about 10 minutes to firm up the dough. This will make trimming and decorating the "fish" easier. Preheat the oven to 375 degrees.
After 10 minutes, remove the "fish" from the freezer, and trim the excess dough all around the edge to create a fish outline with dorsal fins as well as a tail from the extra dough around the salmon. Make a gill with a strip of the trimmed dough, and cut and position a round piece of dough for the eye. Continue decorating as you fancy.
Break the whole egg into a small bowl, and remove about half the white. Beat the remaining egg in the bowl with a fork to make a wash for coating the "fish". Brush the salmon with the egg. Holding the tip from a pastry bag (large end down) at an angle, press it lightly into the "fish" to simulate scales. Bake in the 375 degree F oven for 35 to 40 minutes. Remove and set aside, uncovered, in a warm place while you make the sauce.
Foamy lemon sauce: Whisk together 1 cup of the chicken stock and the potato starch in a saucepan, and bring to a boil. Whisk the egg yolks and the remaining chicken stock in a bowl, then pour into the hot stock, and whisk constantly over medium heat for about 2 minutes, or until the sauce thickens and foams, but don't let it boil. It should reach a temperature of 180 degrees, the temperature needed to cook the eggs and the starch and to thicken the sauce. Whisk in the salt and cayenne, then remove the saucepan from the heat, stir in the lemon juice, and set aside. The sauce should have doubled in volume.
Using 2 long hamburger spatulas, transfer the "fish" to a serving platter. To serve, cut into 1 to 1 1/2-inch slices, and arrange on warm individual plates. Spoon some sauce around the slices, and serve immediately.