Preheat oven to 400 degrees F (200 degrees C). Grease a baking sheet
Heat oil in a large skillet over medium heat. Add spinach; cook and stir until wilted, about 3 minutes. Season with salt and pepper. Drain off any excess liquid; set aside.
Lay the sheet of puff pastry out on a clean surface, and cut into 4 squares. Prick lightly with fork.
In a medium bowl; mix crabmeat, bechamel sauce, shallots, and tarragon until well blended.
Cut prawns lengthwise and open them up to form a butterfly shape. Lay one shrimp in the middle of each pastry square with open side facing up. Stuff each shrimp with an equal amount of spinach and top with a heaping tablespoon of the crab mixture. Fold over pastry to make a triangle, and press the edges to seal. Place on the baking sheet, and brush with beaten egg.
Bake in preheated oven until golden brown, 15 to 20 minutes. Serve warm.
