Preheat the oven to 190`c. In a large saucepan heat the flora cuisine and add the onions, saute for 4-5 minutes to soften. Add the leek and cook for 2 minutes stirring all the time. Add the flour and mustard powder and cook for 1 minute. Add the stock and allow this to reduce by half. Add the cream, reduce the heat and cook for 6-8minutes until the sauce has thickened. Season well, stir in the spinach and cook until the sauce becomes thick and creamy. Remove from the heat and cool, then stir in the lemon zest and juice and chopped dill. Cut the pastry in half and roll each half out on a floured surface to a rectangle, one approx 20cm x 32cm and 0.5cm thick and the other slightly larger and thinner. Cut the salmon into 2cm chunks season well and add to the sauce with the prawns. Mix well to coat. Lay the thicker pastry rectangle on a non stick baking sheet, then spoon the mixture down the center, piling it up high. Mix the beaten egg yolks with a little water to make an egg wash and brush the edges of the pastry with it. Place the other piece of pastry on top, trimming and crimping edges to make a ‘pillow’. Use leftover pastry to decorate the top. Brush all over with the rest of the egg wash and bake in the preheated oven for 25-30 minutes. Remove from the oven and allow to stand for a few minutes before slicing.