Simmer vinegar, fish sauce, palm sugar, and chilli powder in a saucepan until sugar dissolves, then stir in lemon juice.
Combine cabbage, chillies, carrots, ginger, garlic, and seaweed in a bowl and transfer to an airtight container.
Pour hot liquid over the vegetables, seal the container, shake well, and ferment at room temperature for one week.
Thinly slice scallops, arrange on a platter, and season with lime zest, lime juice, and salt.
Fry halibut in oil for 2 minutes, add butter, and fry for 3 more minutes while basting with pan juices.
Plate a portion of kimchi, top with scallops and one halibut steak, then garnish with coriander cress.