Piquant Sauce with Onion, Shallot and Gherkins

• 1½ oz (40 g) butter
• 2 medium onion, finely chopped

Heat a little of the butter in a sauté pan, add the onion and shallot and cook slowly until they begin to soften.

Reduce the vinegar by half and add it, with the meat glaze, to the sauté pan. Cook slowly for 15 minutes, remove from the heat and add the rest of the butter, whisking. Finish the sauce by adding the gherkin and some pepper.