Cook the beans in a large saucepan of lightly salted boiling water for 2 minutes. Drain and immediately refresh under cold water. Pat dry with kitchen paper, then peel off and discard the tough outer skins. Put the beans in a bowl.
Trim the fennel bulb. Cut in half, then crossways into wafer-thin slices. Add to the beans with the watermelon and feta.
Whisk all the dressing ingredients together in a small bowl and season with salt and pepper. Pour over the salad, toss well and serve.