Cook the potatoes (uncovered) for approx. 30 mins. in boiling salted water until soft. Drain the potatoes and leave to stand for approx. 5 mins. Transfer the potatoes to two baking trays lined with baking paper, crush with the back of a spoon. Mix the oil, garlic and rosemary, season and spread on top of the potatoes.
Approx. 45 mins. in an oven preheated to 200°C (convection). Remove, serve the potatoes on a platter.
Heat the oil in a pan. Add the onions and sauté for approx. 5 mins. Add the vinegar, reduce completely, season. Salt the crème fraîche, stir in half of the chives, spread on top of the potatoes. Top with the onions and the remainder of the chives.