Wash the prawns, peel, remove the intestine and finely chop 4 of them.
Peel and chop the shallots and garlic.
Wash the ginger and lemongrass and chop very finely.
Fry the shallots and garlic in hot oil, add the prawns pulp and cook for 2-3 minutes. Add the ginger and lemongrass. Season with chilli, soy sauce and fish sauce, and set aside.
Bring 1l water to the boil in a pan with sea salt, lemon and bay leaves. Cover and simmer for 5 minutes. Add the prawns for 2 minutes, then remove, rinse under cold water and drain. Peel the prawns, leaving the tail intact.
Let the pastry sheets thaw. Place a teaspoon of the chopped prawn mixture on each and spread out in the middle.
Brush the pastry edges lightly with egg white.
Place a whole prawn on top of the pastry so that the tail sticks out on one side.
Fold the pastry from the lower corner tightly over the prawn.
Turn the adjacent corner inwards and turn the last corner over the prawn so that it is wrapped and only the tail can be seen.
Heat a deep fat fryer or deep pan half-filled with oil to 170°C. Fry the wrapped prawns until golden brown. Drain on kitchen paper. Serve with a sweet and sour chilli dip.