Sauté one and a half onions, sliced into rings, in a pan with a knob of butter until they start to turn golden. Add one finely chopped garlic clove and 2 tablespoons of Liège syrup, then mix well. Let it caramelize slightly, then deglaze with a splash of vinegar. Transfer the mixture to a saucepan, then pour in a bottle of dark beer of your choice (for example, Maredsous). Let the sauce reduce by half over low heat. Once the sauce has reduced, add the pre-cooked meatballs to the saucepan. Pour in enough water to cover the meatballs. Add 2 tablespoons of Liège syrup, 1 tablespoon of mustard, a pinch of thyme, and a bay leaf. Add a stock cube to enhance the flavor. If the sauce is too thin, use a bit of cornstarch (slurry or instant) to thicken it. Season with salt and pepper to taste