Snip a few handfuls of watercress into a large bowl. Halve your cucumber lengthwise. Finely slice.Quarter the your fennel. Thinly slice. Add to the watercress.
Pop half your feta into a food processor or blender. Pick the mint leaves from the stalks. Add half of them along with 4 tbsp cold water. Blend till smooth, adding a little more water, if needed.
Pour the feta dressing over your salad. Mix through. Arrange on plates. Crumble the remaining feta over. Scatter the remaining mint leaves on top. Finish with 1 tbsp olive oil and a little freshly ground pepper.