Preheat oven to 400°F / 200°C / Gas Mark 6.
Scrub potatoes and rinse under cold running water, then drain well.
Slice potatoes lengthwise. In a medium mixing bowl, toss with duck fat, salt and pepper.
Place potatoes, cut side down, on baking sheet and roast for 22 - 25 minutes, or until easily pierced with a knife.
Allow potatoes to cool slightly (enough to handle them), then toss in a medium mixing bowl with whole grain mustard, sherry vinegar, parsley and chives.
Adjust seasoning, if needed, and serve warm.