Preheat the oven to 180˚C.
In a large bowl, combine the crème fraiche, most of the butter, egg, garlic, onions, the grated cheese and the chives. Stir in the mash potatoes. Season well with salt and pepper.
Transfer the mixture in a gratin dish.
In a second bowl, combine the breadcrumb, the rest of the chive and the leftover of grated parmesan. Season well.
Sprinkle the mixture over the mash potato gratin. Add a few extra dots of fresh butter if desired.
Place in the oven and bake for 30 minutes until piping hot and golden brown on the top.
Remove from the oven and serve immediately.