Season the trout with salt, pepper, and 1 tablespoon of the tarragon. Reserve in the refrigerator.
To make the fish mousse: Puree the pike in a food processor. Add 1 egg, mix well, and with the motor running, add the cream. Refrigerate for 2 hours.
Divide the pastry dough in half and roll out each piece so it is approximately 1/3 inch thick. Cut the pastry into 12 equal pieces that are all slightly larger then the fish fillets. Place half of the pastry pieces on a baking sheet. Place 1 fish fillet in the center of each piece of pastry, skin-side down. Spread the chilled mousse over the fillets and cover with another fillet, skin-side up. Lightly beat the remaining egg for egg wash. Brush the pastry around the edge of the fish and cover with a second sheet of pastry. Press the layers of pastry around the edges of the fish to seal. Trim the pastry following the outlines of the fish, and use the leftover dough to decorate the fish with scales, an eye, and gills. Chill in the refrigerator for at least 30 minutes.
Preheat the oven to 375 degrees F.
When ready, bake the fish for 15 to 20 minutes, or until the pastry is golden brown and the center of the mousse is hot (when you insert a skewer in the center for 1 minute, it should be hot when touched to the lip).
Sprinkle fish with remaining 1 tablespoon tarragon.
Serve the fish with Chive Butter Sauce.
Chive Butter Sauce:
In a saucepan, combine shallots, tarragon, wine, and thyme. Reduce until only 1/4 cup of the liquid remains. Add the cream and reduce until the mixture thickens slightly. Slowly whisk in the butter, 1 small piece at a time. Add the lemon juice, salt, and pepper, to taste. Strain the sauce into a small saucepan and stir in the chives.
Recipe courtesy Wolfgang Puck, "Modern French Cooking for the American Kitchen" Houghton Mifflin, 1981.