Heat 1½ tsp neutral oil in a deep pan over medium heat. Add ginger, garlic, onion, and chilli. Sauté 2–3 minutes until fragrant and softened.
Add coconut milk, hot water, stock cube, soy sauce, coriander powder and black pepper powder. Bring to a gentle simmer.
Add fish pieces in a single layer. Cover and cook on low for 5–7 minutes, until the fish is just cooked and flakes easily. Do not over-stir.
Finish with lemon juice, a drizzle of thick coconut milk, chilli oil, and garnish with fresh coriander leaves.
Serve hot with steaming white rice.
