How to Make It
In a saucepan, boil the potatoes in water to cover until tender, about 15 minutes.
Meanwhile, in a small skillet, toast the mustard seeds over moderate heat until they pop, about 3 minutes. In a small bowl, mix the butter with the parsley, shallot, lemon juice and mustard seeds. Season generously with salt and pepper.
Drain and halve the potatoes lengthwise. In a large dish, toss the potatoes with the shallot butter and serve.