Bring a large pot or wok of water to a rapid boil. Add the mussels and cook until the mussels slightly open. Remove from the pot and set aside.
In a hot wok, add oil and then lemongrass. Cook the lemongrass until fragrant, now add Asian shallots and garlic and stir-fry for 1 minute.
Pour in sugar, coconut milk, and fish sauce. Stir and bring to the boil. Once boiled, add the mussels and toss for 1 minute.
Add pepper, Vietnamese mint, and chile.
Toss for a further minute and serve.