Cut the potatoes in half and cook in a large saucepan of boiling water for 10 minutes, until tender. Drain, then briefly cool.
Place the potatoes in a large bowl. Add the witlof, parsley, tarragon and walnuts, and toss to combine.
Combine the lemon juice, grated horseradish and crème fraîche or sour cream in a small bowl. Season with sea salt.
Pour the dressing over the salad and toss to combine.
Adding the dressing while the potatoes are still warm means they will absorb the flavours of the dressing. Fresh horseradish roots can be grated and frozen. Grating releases their pungent volatile oil.
Per serving: 236 calories, 5 g protein, 16 g fat (8 g saturated fat), 18 g carbohydrate (2 g sugars), 3 g fibre, 96 mg sodium