Sauce Robert

Melt about a third of the clarified butter in a heavy-bottomed pot over medium heat. Add the mirepoix and cook, stirring occasionally, until deeply browned and sweet-smelling — 12 to 18 minutes. Build a rust-colored fond on the bottom of the pot; that caramelization is the backbone of the espagnole base.

Melt the remaining butter into the cleared part of the pot, sprinkle in the flour, and cook the roux to a deep chestnut brown, stirring almost constantly — 10 to 20 minutes, until it smells nutty and toasty. Stir in the tomato paste and cook it out for 3 to 5 minutes until it darkens to brick-rust and smells sweet rather than sharp. Go low and patient; brown roux scorches in a heartbeat.

Pour in the brown stock a few ladles at a time, whisking out lumps before adding more. Add the bouquet garni, bring to the barest simmer, and hold 2 to 3 hours, skimming the fat and froth from the surface and seasoning lightly with salt near the end. When reduced by about a third and coating a spoon, strain through a fine sieve. This is your base espagnole.

In a separate clean saucepan, melt a little clarified butter over low-medium heat and add the finely diced onion. Sweat it gently, stirring often, until soft, translucent, and just beginning to turn golden at the edges — 8 to 12 minutes. The goal is sweetness and tenderness without hard browning; keep the heat low so the onion melts rather than crisps.

Pour the dry white wine over the sweated onions and raise the heat to bring it to a brisk simmer. Reduce, scraping up any fond, until the wine has cooked down to a syrupy glaze barely covering the onions and the sharp alcohol smell has cooked off — 5 to 8 minutes. This reduction concentrates the wine's acidity into a bright backbone for the sauce.

Pour the strained espagnole into the onion and wine reduction and simmer gently together for 8 to 10 minutes, skimming, until married and smooth. Then pull the pan completely off the heat and let it stop bubbling before whisking in the Dijon mustard. Never let the sauce boil after the mustard goes in — boiling mustard turns harsh and bitter. Taste, adjust the salt, and serve warm.