Cook diced potatoes in salted water. When soft but still firm, use a slotted spoon to transfer immediately to ice-cold water. Drain well and transfer to a bowl. Mix together chopped tarragon, lemon juice, zest, lime juice, mustard and olive oil. Let infuse. Strip the leaves off the chard stems. Dice the stems, chop the leaves fine. Heat the vegetable oil in a skillet and add the stems. Stir-fry briefly, then add the leaves to wilt. Season with salt and pepper. Transfer to a plate and let cool completely. Chop the chard mixture smaller with a kitchen knife (fine enough to mix well with the potatoes). Mix together potatoes and tarragon dressing. Spoon through chard and finish to taste.