This sauce is SO good I can almost eat it all on it's own - it's very tangy and rich in flavour and it's the best accompaniment (at least according to ME) to what we here in South Africa call "pap", which is traditionally eaten as a side dish at a bbq. It's sort of a porridge made from maize meal and there are a variety of tomato & onion-y sauces usually eaten with this "pap" and it's called "pap en sous" (porridge & sauce) - but this red sauce is WAY better than those tomato-y based sauces. :)
When we're not having a bbq with people over, I serve this sauce with cous cous - I think it's a little healthier than the refined maize meal.
This recipe makes a large amount of sauce, but it freezes really well in a sealed tub - when you need some all you have to do is dish out how much you need and heat it up in the microwave.
250 g butter
1 tsp mustard powder
30 ml worcestershire sauce
125 ml sugar
125 ml vinegar
250 ml tomato sauce (what some of you call ketchup I think)
1 medium onion, finely chopped
1 finely chopped green pepper (what some of you call bell peppers I think)
Dump everything in a pot, heat and stir until the butter is melted (if you're using frozen butter) and all ingredients are mixed together, then cook on medium to high heat for about 10 minutes. No need to boil it at any stage.