How to Make It
Preheat water with immersion circulator in large stockpot to 185°F according to manufacturer’s instructions. Fold over top of 1 large vacuum sealer bag about 1 1/2 inches to keep top of bag clean. Carefully combine potatoes, mustard, oil, vinegar, sugar, salt, and pepper in bag; massage to ensure potatoes are evenly coated. Using a vacuum sealer, remove air and seal bags tightly according to manufacturer’s instructions.
Place bags in preheated water, and cover top of stockpot with aluminum foil to reduce evaporation. Cook until potatoes are tender (you should be able to break apart potatoes through the bag), 3 to 4 hours.
Remove bag from water, and let stand 10 minutes. Open bag, and transfer potato salad to a serving dish. Sprinkle with chives.