-
1
Optional infusion: place the milk in a small saucepan with 1 tablespoon of lightly crushed mustard seeds. Warm gently, remove from heat, and steep for 20 minutes. Strain, discarding the seeds. The milk will carry a quiet, round mustard base beneath the finished sauce.
-
2
Melt the butter in a heavy saucepan over a moderate flame. Add the flour and stir continuously with a wooden spoon for 1–2 minutes until the roux smells faintly of biscuit and loses its raw, floury scent — it should not colour.
-
3
Add the milk in a steady stream, whisking constantly to prevent lumps. Bring to a gentle simmer and cook, stirring, until the sauce is of medium thickness — it should coat the back of a spoon and hold a clear line drawn through it with a finger.
-
4
Season with salt and white pepper. Remove the pan from the heat and allow the temperature to fall below 70 °C — this is where the dish lives or dies: mustard added to a sauce still on the flame will lose every volatile compound that gives it warmth and character, leaving only a flat bitterness behind.
-
5
Whisk in the Dijon mustard one tablespoon at a time, tasting after each addition. The finished sauce should show warmth in the nose and a gentle burn at the back of the throat — distinct mustard presence, not aggression. If using whole-grain mustard, add it now alongside the Dijon for texture without visual disruption.
-
6
Add a squeeze of lemon juice to lift the sauce. If the acidity is sharp beyond comfort, stir in the tablespoon of cream. The sauce should be pale yellow-cream, glossy, and pourable — tilt the pan to confirm it moves freely.