Heat 2 tablespoons of the butter in a sauté pan over medium heat (preferably the same pan you used to cook the meat you are saucing), and sauté the shallots or onions until softened but not brown, about 2 minutes. Add the stock, wine, vinegar, and mustard and bring to a boil over high heat, stirring regularly and making sure to scrape up all the brown bits. Continue to cook until the liquid is reduced by half and lightly thickened, 3 to 5 minutes. Remove the pan from the heat, and whisk in the remaining 2 tablespoons of butter a bit at a time to thicken. Taste and season with salt and pepper. Serve immediately, spooned over hamburgers (call it steak haché, in the French manner) or steaks.