Trim the top of the mushroom stalks level with the caps, drizzle the gills with the sesame oil and ketjap manis or soy sauce, then sprinkle with the ginger, garlic and coriander. Top each with a slice of tomato, a piece of butter and a little pepper.
Cut the pastry into 4 pieces, roll out one piece thinly on a lightly floured surface to a rough-shaped 18–20 cm (7–8 inch) circle, or large enough to enclose the mushrooms (this will depend on how big they are, so make a little bigger if needed).
Place a mushroom on top of a pastry circle, brush the edges with beaten egg, then lift the pastry up and over the top of the mushroom, pleating the pastry as you go and pinching the ends together in the centre of the mushroom to completely enclose it. Repeat to make 4 parcels and put them on to a buttered baking sheet.
Brush the parcels with beaten egg and sprinkle with sesame seeds. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 25 minutes until golden brown. Transfer to serving plates and accompany with stir-fried vegetables and soy sauce.