1. Combine coconut milk, fish sauce, lime leaves, lime juice, ginger, garlic and lemon grass in a large saucepan over a high heat. Bring to the boil. Reduce heat to medium. Simmer for 15 minutes or until liquid thickens slightly.
2. Reduce heat to low. Add the fish and simmer, covered, for 3 minutes or until just cooked through.
3. Use a slotted spoon to divide fish among 4 serving dishes. Spoon over some of the poaching liquid and top with cilantro, green onions, red onion, and jalapeño slivers. Serve.