Recipe: barbecue relish

This is for Chris and the full recipe can be found here.  It looks as if it has been freely reproduced in various places so I don't feel too bad about reproducing it here too. especial;ly as it has changes.
http://slimmingworldrecipesblog.blogspot.com/2016/02/bacon-burger-with-bbq-relish.html
The bbq relish is useful to make on its own for all sorts of things and here it is, adapted to reflect what I actually did.

You need:
Low cal cooking spray
1 onion, peeled and chopped (I actually sliced mine very thinly)
1 tsp mild chilli powder (I used chilli puree) - or you could use a bbq spice mix, that would work well.  Schwartz does a nice, syn free one.
a squidge of garlic puree (not in the original recipe but how can you make a bbq sauce without garlic?)
225g passata (or use chopped tomatoes, blend and sieve out any seeds)
sweetener (it says 3 tbsp - I added to taste as it seemed rather a lot)
4 tbsp Worcestershire sauce
1/2 tsp mustard powder
salt and pepper
about 2 tbsp tomato puree (not in the original recipe)
1 tbsp balsamic vinegar (optional and not in original recipe)
and I also added some soy sauce just because I like it!

Method.
Cook the onions slowly in some spray oil until going soft, then add the chilli and garlic and cook for a bit longer to 'cook it out'.
Stir in the remaining ingredients (I didn't add the sweetener until the end so I could taste and add, taste and add, until it was 'right'), bring to a boil and let it simmer gently until it all thickens and goes gooey.  It tends to splat and I used my anti-splash guard - it keeps the hob cleaner!

Check the seasoning (and sweetener) and adjust as needed.  I reckon a splash of lime juice might be nice too and I'll try that next time.

Use hot or cold and it keeps for at least a week (mine got finished off by then) in a jar in the fridge.  I reckon it will last longer than that but can't be 100% definite without trying.

The whole recipe with coleslaw and SW cooked chips; you can't really see the relish but I could certainly taste it.  Yum!