Preheat oven to 400 degrees F. Boil potatoes until firm/tender. Drain well, and then place in a bowl. Add remaining ingredients and gently toss. Place potatoes in a single layer on a baking tray. (Cover and refrigerate now if roasting later). Roast 25-30 minutes, turning once halfway through cooking to ensure even browning.
Recipe Options: An equal amount of white-skinned potatoes or small baking potatoes could replace the red. Regular Dijon mustard could replace the grainy. A tsp. of dried rosemary could replace the fresh.