Cut the lemongrass and ginger into slices, finely dice the shallot. Sweat lemongrass and shallots in a little vegetable stock, add ginger and deglaze with coconut milk, reduce the heat and let stand on a low level for about 30 minutes, season with salt, chilli, curry paste and turmeric.
Sear the salmon in a pan with garlic and set aside.
Pass the soup through a sieve and bring to the boil again briefly. Just before serving, refine with lime juice and mix with the cold butter.