Melt butter in a large frying pan over medium heat. Saute onions until tender. Stir in dark brown soft sugar, ketchup, vinegar, mustard, Worcestershire sauce and pepper sauce. Simmer, stirring occasionally, for 20 minutes.
Place sausage in a slow cooker with the sauce and onions. Cook on low for 4 to 5 hours. The longer it simmers the better!
Kielbasa are smoked Polish sausages and are available at Polish delis. If unavailable, substitute in Mattessons smoked pork sausage.
Very tasty sticky and sweet but with some punch. Used the mattesons smoked sausage. Will be making again. Couldn't be bothered to simmer the sauce just brought it to boil and chucked it in the pot and it worked just fine - 09 Jan 2013