Peel and crush the garlic.
Peel and roughly chop the ginger.
Roughly chop the lemongrass.
Slice the chili.
Using a mortar and pestle (or a small food processor), grind the garlic, ginger, chili and lemongrass into a paste.
Peel the carrot and thinly slice as well.
Heat the cooking oil in a wok or frying pan. Stir fry the carrot slices for half a minute. Scoop out and set aside.
Add the spice paste to the oil. Cook over medium-low heat until aromatic and the solids separate from the oil.
Dump the soaked and de-bearded mussels into the pan. Turn up the heat to high. Splash in fish sauce. Stir fry for a minute or two or just until the shells open.
Add the carrot slices back to the pan. Toss. Taste the sauce. Add more fish sauce, if needed. Add the scallions. Toss a few more times. Turn off the heat.
Serve the lemongrass tamarind mussels immediately.