Heat the chicken broth. Add the curry paste, lemongrass, kaffir lime leaf and ginger and simmer for about 10 minutes. Filter the broth through a sieve, add the diced tomatoes and coconut milk, put in the scallops and let them steep for about 5 minutes. Season to taste with salt, pepper and possibly a little lime juice and a pinch of sugar. Serve the broth with 3 mussels each on a plate, garnished with coriander leaves.