Boil, peel and slice the potatoes. Melt the butter. Add the onion and diced ham and sauté until translucent. Sweat the flour in it. Deglaze with stock and milk. Stir constantly so that no lumps form. Let the sauce simmer over low heat for about 10 minutes. Season to taste with salt and pepper. Then stir in the crème fraîche. Heat the potato slices in the sauce. Sprinkle the finely chopped parsley over the top before serving.