2 cups balsamic vinegar
1/2 cup chopped yellow onion
2 to 3 teaspoons whole peppercorns
3 cups chicken broth (I use Campbell's double strength, undiluted)
3 cups beef stock
1/2 cup butter
In a medium-sized, heavy-bottomed pot (about 8-quart capacity), add the vinegar, chopped onion and peppercorns. Bring to a boil, reduce to a simmer and cook until reduced to a syrup consistency (you will have about 1/2 cup of liquid). Add the chicken broth and beef stock, bring to a boil again, then simmer until reduced to a sauce consistency (you will end up with about 1 cup).
Strain sauce through a wire sieve, pressing down with a wooden spoon or spatula to force softened onions through sieve for extra flavor.
The sauce may be refrigerated for several days or frozen for several months at this point. When ready to finish sauce, bring it to a simmer then whisk in the butter, 1 tablespoon at a time, until all butter has been incorporated. Keep hot until ready to serve.
Makes about 11/2 cups sauce.
— Recipe adapted from Chef Michael Chiarello, Napa Valley, Calif.