Dipping Sauce: In a mortar pound the garlic with a good pinch of salt. Add the chillies and pound well to create a paste.
Add lime juice, fish sauce and palm sugar. Taste and adjust to your liking. The sauce should be spicy first, then sour, then salty, then sweet.
Mussels: Put the mussels, lemongrass and Thai basil into a big saucepan over high heat. Add a 1/4 cup of water to start the steaming process.
Cover with a tight fitting lid. Steam in its own liquid for about 8-10 minutes or until the shells have opened. Shake the saucepan every now and then during that time to spread the mussels, lemongrass and Thai basil around.
Discard any unopened mussels. Serve immediately in a big bowl with the dipping sauce on the side.