Combine prawns, fish and scallops in a mixing bowl. Sprinkle lightly with a dash of salt, sugar, pepper and a little sesame oil. Mix and set aside.
Heat butter and olive oil in a saucepan and stir-fry prawns, fish, scallops and button mushrooms briskly. Remove and drain on absorbent kitchen paper, leaving the remaining butter in the saucepan. Add in onion and garlic. Fry until onion turns soft. Dish out, drain well and set aside.
For the sauce: melt butter in a saucepan. Turn to a medium-low heat, then stir in flour. Use a wooden spatula to stir continuously until bubbles appear. Remove saucepan from heat and gradually add in the combined milk mixture.
Return saucepan to the stove and heat until the whole mixture gradually boils and thickens. Remove from heat and stir in cheese and seasoning.
Combine cooked seafood, onion and cream sauce in a mixing bowl. Mix well, then leave to cool completely. Turn this seafood mixture into an oven-proof pie dish. Sprinkle with parsley.
Roll out the puff pastry to a size that is big enough to cover the dish as well as have 3cm to 4cm of pastry hanging down the sides of the dish. Fold the overhanging pastry onto the edges of the dish. Press to seal.
With the remaining pastry, roll out and cut into desired shapes such as fish, stars, etc. Dot a little beaten egg mixture on the cut-out shapes, then place on the pie to form a decorative pattern.
Cut small slits on the pie. Brush pie with beaten egg to glaze.