Preheat oven to 200 degrees C. Grease a baking tray.
Heat oil in a large frying pan 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 then cut into 4 squares. Prick lightly with fork.
In a medium bowl; mix crab meat, bechamel sauce, eschalots and tarragon until well blended.
Cut prawns lengthwise and open them up to form a butterfly shape. Lay one prawn in the middle of each pastry square with open side facing up. Stuff each prawns with an equal amount of spinach and top with a heaping tablespoon of the crab mixture. Fold over pastry to make a triangle then press the edges to seal. Place on the baking tray then brush with beaten egg.
Bake in preheated oven until golden brown, 15 to 20 minutes. Serve warm.
The parcels can be frozen prior to baking for up to 2 weeks before you need them. Thaw in refrigerator for 4 to 5 hours before baking.