In a heavy saucepan, melt the butter over medium heat. Add the onion and saute for a couple of minutes, until the onion is softened.
Mix in the remaining ingredients, reduce the heat to low, and cook the mixture until it reduces by about half, approximately 40 minutes. Stir frequently.
Serve the sauce warm or chilled. It keeps, refrigerated, for a couple of weeks.
Recipe by Ultimate BBQ Sauce Recipes! at http://www.bbq-sauce-recipes.com/citrus-based-sauces/330/lime-mint-bbq-sauce/