Using a mandoline slicer (available at home-cooking shops), slice fennel bulbs crosswise and as thin as possible. Peel and cut persimmons into small wedges. Using a knife, peel oranges, sectioning and removing all membranes over a bowl to catch juice. Reserve juice that remains.
In a salad bowl, add lemon and orange juice, salt and pepper. Mix until salt dissolves. Slowly add olive oil. Add fennel, persimmon, orange sections and watercress tops. Sprinkle with fennel seeds. Toss and serve.
