Cook the beans in boiling salted water for 5-6 minutes, until just tender, then drain and refresh under cold running water. Shake well to remove excess water, then pat dry with kitchen paper. You can do this the day before and keep the cooked beans in the fridge, in a bowl covered with cling film.
Heat 1 tablespoon of the oil in a small pan, add the garlic and fry quickly until crisp and lightly golden. Tip into a bowl with the oil from the pan and leave to cool. This can be done the day before and covered with cling film and chilled.
On the day, whisk together the remaining oil, vinegar, mint, salt and pepper. Pour over the beans and mix well. Tip into a serving bowl and scatter over the garlic in its oil.