Put the garlic, shallots, lemongrass and chilli in a saucepan with a small amount of butter. Gently soften over a medium heat.
After 5 minutes, add the fish stock, coconut milk and kaffir lime leaves.
Simmer for a further 5 minutes. The broth is now ready and can be kept in the fridge for up to 3 days.
Put a large saucepan over a high heat. Once hot, add the mussels and clams, then pour in the broth and cover.
Steam for a couple of minutes until the shells have opened.
Divide between 4 bowls and garnish with the spring onion, coriander and mint leaves, and serve with wedges of lime.
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