Place the garlic, red onion, galangal, lemon grass, turmeric, paprika, fish sauce and brown sugar in a blender and process until finely blended. Add the coconut milk and process again until thoroughly blended.
Transfer the coconut mixture to a heavy-based pot. Squeeze the Kaffir lime leaves in your hand first, to release the oils and aroma, then add to the pot. Bring the coconut mixture to the boil then reduce to a gentle simmer and cook for 10 minutes to allow flavours to infuse and until mixture has reduced slightly.
Add the fish chunks to the coconut mixture and poach gently for 4–5 minutes. Add lemon juice and stir. Transfer to a large shallow serving bowl and serve immediately.