Scrub the mussels well and remove the beards (remove any broken or open mussels). Chop the garlic and the chilli finely. Meat the peanut oil in a frying pan and fry the garlic and chilli mixture until golden. Add the nam pla followed by the mussels. Cover. When the mussels have opened (this will take only a few minutes), discard half of the shells, place the remaining shells on a serving dish. Do not use any mussels that have not opened properly. Add the mint to the mussel juice and pour over the mussels.