Add the potatoes, rosemary and a generous pinch of salt to a large pot. Add enough water to cover the potatoes. Bring to a boil and cook until the potatoes are tender and can be easily pierced with a fork, about 15 minutes. Reserve 1 cup of the potato cooking water. Drain the potatoes.
Cut the potatoes in half. (Set 1 half potato per portion aside, cut a sliver off the rounded part of the potato so it sits flat). Mash the rest of the potatoes with a fork or potato ricer.
Whisk the mustard, creme fraiche, rosemary, mashed potato, salt and pepper until smooth. Add more creme fraiche if needed for creaminess.
Preheat the oven broiler on high.
Transfer the filling to a piping bag or zip-top bag. (if using a zip lock bag slice a 1/2-inch opening in the corner of the bag).
Pipe the mashed potato onto the boiled potato halves. Place under the oven broiler to get the tops golden brown.