In a large pot, cover the potatoes with water, add a large pinch of salt and boil until just tender, about 25 minutes. Drain and let cool slightly.
In a small skillet, toast the fennel seeds over moderate heat until lightly browned, about 40 seconds. Let cool, then crush the seeds to a coarse powder in a mortar. Transfer the fennel to a bowl and stir in the mustard, mayonnaise, vinegar, Worcestershire, shallot and garlic.
Quarter the potatoes and transfer them to a large bowl. Add the mustard dressing and toss well. Season with salt and pepper. Serve warm or chilled.