Clean and slice potatoes into bite sized rounds and boil until fork tender. Squeeze out the pulp from the head of garlic. Make vinaigrette by whisking garlic, mustard and vinegar in a bowl and then slowly whisking in olive oil, season with salt and pepper and add mustard seeds at the end. While potatoes are still warm and drained, toss with vinaigrette. Just before serving add onions, herbs and watercress.