1 small onion
3/4 - 1 cup Sauce Espagnole
1 Tbl or to taste Dijon mustard
In advance, prepare the Sauce Espagnole. Dice the onion finely.
Sweat the onions down in 2 Tbl of butter. Deglaze the pan with white wine. Scrape the bottom of the pan. Reduce the wine by about 1/2. Throw in the Sauce Espagnole. Adjust seasonings to taste. Reduce the sauce until it coats the back of a spoon. Stir in the mustard, then cut off the heat.
One shallot may be substituted for the onion.