Place the eel and seasoned flour into a food bag. Seal and shake the bag to coat the eel in the flour. Remove the eel from the bag and shake off any excess.
Heat half the oil in a large frying pan over a medium heat. Add the floured eel and fry, in batches if necessary, for 1-2 minutes on each side, or until browned on all sides. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper.
Wipe the pan clean with a sheet of kitchen paper. Heat the remaining oil over a medium heat, then add the garlic and fry for 1-2 minutes, or until softened but not coloured.
Add the vinegar to the pan, then season the garlic, to taste, with salt and freshly ground black pepper. Remove the pan from the heat and set aside until the contents have cooled.
Stir the mint leaves into the cooled vinegar and garlic mixture.
Place the drained pieces of fried eel into the bottom of a glass serving dish. Pour over the cooled marinade mixture. Cover and chill in the fridge for 5-6 hours, or preferably overnight, turning the pieces of eel in the marinade from time to time.
Serve cold with a green salad and slices of good-quality crusty bread.