Preheat oven to 400°F. Prick potatoes with a fork and roast for 30 minutes. Remove and cool to touch. Turn oven heat up to 500°F.
Combine mustard, garlic, thyme, lemon juice or vinegar, oil, salt and pepper in a large bowl.
Cut potatoes lengthwise into 6 wedges each. Add potatoes into the bowl with the dressing and toss to coat well. Spread potatoes in an even layer on a baking sheet with the wedges lying flat. Roast 20 minutes, turning once. Top with chives and chopped cress to serve.