Peel the garlic and squeeze lightly with a knife.
In lightly salted water, cook the potatoes with the rosemary and garlic for about 15 minutes. Drain, discard the rosemary and garlic.
Meanwhile, chop the shallot, then gently sauté it in the oil until soft, then set aside.
Put the potatoes in a serving dish and pour over the onion & oil, then stir in the mustard, vinegar and horseradish. Lightly crush the potatoes with a fork while combining. Season with salt and pepper, then fold in the chopped herbs.
Serve warm or at room temperature.