Bring a large saucepan of salted water to a boil. Add the potatoes and pars-nips and cook for about 12 minutes, until they are al dente. Drain in a sieve and let the liquid evaporate. Slide them into a baking dish.
Melt the butter in a saucepan, stir in the flour, and cook for about 3 minutes. Stir in the milk and keep stirring—with a whisk—