Preheat oven to 220°C. Place ginger, garlic, sweet chilli sauce, soy sauce and mirin in a small bowl. Whisk to combine.
Cut four 40cm lengths of foil. Place 1 fish fillet in centre of each piece of foil. Spoon over marinade. Top with onions and lemon slices. Fold foil edges and turn upwards to seal and form parcels. Place on a baking tray. Bake for 15 to 20 minutes or until fish is just cooked through.
Meanwhile, one-third fill a wok with water. Bring to the boil. Place pumpkin in a steamer basket. Cover and steam over simmering water for 10 minutes or until almost tender. Add bok choy. Steam both for 5 minutes or until tender.
Remove fish from parcels and place in shallow bowls. Pour over any juices. Serve with steamed pumpkin and bok choy.