According to renowned Lyonnaise chef Paul Bocuse, "In Lyon we put onion in almost everything." French onion soup originated in Lyon, as does this simple and versatile onion-scented sauce. It's superb with mashed potatoes or roasted vegetables, grilled or roasted beef, pork, lamb, game, or poultry.
Melt the butter in a saucepan over medium heat. Stir in the onion and cook until the onion turns golden, 5-10 minutes. Add the wine and the vinegar, bring to a boil, and cook until the liquid is reduced by 3/4, to about 3 tablespoons.
Season the sauce to taste with salt and pepper and then add the parsley.
Traditionally, Sauce Lyonnaise was strained to remove the onion before serving. We've left the onion in for maximum flavor, but feel free to strain the sauce if you prefer it smooth (do it just after seasoning in step 3, before adding the parsley).