Mix miso paste, sugar, soy sauce, sake, and mirin in a large bowl.
Coat halibut in marinade and refrigerate for 4 hours or overnight.
Pulse prawns, soy sauce, mirin, and ginger in a processor, then stir in spring onion.
Place a teaspoon of prawn mixture onto the center of each wonton wrapper.
Brush edges with egg wash and fold into triangles to seal.
Shape dumplings by joining the base corners of the triangles and chill.
Preheat the grill to its highest setting.
Place halibut on an oiled tray and grill for 5-6 minutes, turning once.
Boil the dumplings in salted water for 2-3 minutes then drain on paper.
Simmer stock, sake, soy, ginger, sugar, and salt in a saucepan.
Add mushrooms, leek, and bok choi and simmer for 2-3 minutes.
Add dumplings to the broth to warm through.
Serve dumplings and broth in bowls, topping with shiso leaves, halibut, and shichimi.