In a heavy-bottom saucepan, melt butter. Whisk in flour, stirring constantly, about 2 min. Mixture will bubble. Do not let it brown. Whisk in hot milk, making sure there are no lumps and mixture is smooth. Continue to stir as sauce thickens. Add in teaspoon of Maille Dijon mustard. Bring to a boil. Add salt and pepper, lower heat, and cook, stirring for 2–3 min. Remove from heat. To cool this sauce for later use, cover it with plastic wrap. Set aside.