Place the sliced onion and figs in a small bowl and add the sherry vinegar. I like to massage the vinegar into the onion slices to soften them up a bit. Set aside for 5 minutes.
Blanch the beans in a bowl of boiling water for 2 minutes,(they will still have a bit of crunch), then drain and run under cold water for a few seconds. I like the beans crunchy.
Put the beans on a plate or in a shallow bowl. Remove the figs and onions from the liquid and add to the beans. Reserve the vinegar.
Whisk the olive oil and vinegar together with salt and pepper to make a dressing.
Toss the beans in the dressing, then sprinkle the hazelnuts and mint on top. Serve immediately.
Notes
For this salad, you can prepare the beans, figs and onions a day ahead. Just before serving, add the hazelnuts and mint.