Sauce Charcutière and Sauce Robert

Ingredients

• For sauce Robert
• 1 medium onion, finely chopped
• 150 ml dry white wine
• 400 ml demi-glace
• 2 tbsp Dijon mustard, added off heat only
• 30 g cold unsalted butter, cut into small cubes for mounting
• For sauce charcutière (additional)
• 60 g cornichons, cut into fine julienne; blanched briefly if milder sharpness is preferred, raw for full contrast

Method

1. 1
   Sweat the finely chopped onion in a little butter over moderate heat until softened and translucent. Add the white wine and reduce almost completely — nearly dry — so that the wine's acidity is concentrated into the onion base.
2. 2
   Add the demi-glace. Bring to a gentle simmer and cook for 10 minutes, stirring occasionally, until the sauce has a smooth, cohesive body and the onion has surrendered entirely into the liquid.
3. 3
   Pass the sauce through a fine strainer, pressing the solids to extract all liquid. The result should be a glossy, deep brown sauce of coating consistency.
4. 4
   Remove the pan from the heat and allow it to cease boiling. Stir in the Dijon mustard — 1 tablespoon per 200 ml of sauce — and mount with the cold butter cubes, swirling until fully incorporated. This is where the dish lives or dies: mustard added above 70 °C loses its allyl isothiocyanate compounds irreversibly, and no quantity of mustard added afterward can restore the sharp, clean note that defines both sauces.
5. 5
   For sauce charcutière only: fold the fine julienne of cornichons into the finished sauce Robert. Do not return the sauce to a boil and do not strain again. The cornichons should retain their texture and either their raw sharpness or, if briefly blanched, a moderated acidity.