I see a lot of references to commercial versions in the UK. A Youtube video takes a pre-made skinless sausage and adds caramelized red onion, minced fresh sage, and minced fresh rosemary to make sausage rolls.
Here is a site that discusses Sainsbury's Pork and Caramelized Red Onion Sausage, and has a picture of the ingredient list. You can reverse engineer a decent attempt from the ingredient label.
http://bangersandsausages.blogspot.com/2010/01/sainsburys-taste-difference-british.html
It appears to be:
75% pork and fat
15% caramelized red onion mixture
10% remaining ingredients
The caramelized red onion mixture appears to be 27% red onions, white/yellow onions, garlic, and finished in a glaze of brown sugar, red wine, white wine, water, vinegar, cornflour, salt and pepper.
The 10% remaining ingredients is composed of breadcrumbs, salt, fresh parsley, white pepper, dried onion, sodium metabisulfite (to prevent fermentation?), nutmeg, sage, coriander, and ascorbic acid (color?).
I'd lose the sodium metabisulfite and ascorbic acid, myself. Probably drop the dried onion, too, but maybe not.
If I were going to try to approximate the Sainsbury product, I'd caramelize the finely chopped red and white/yellow onions and minced garlic in oil, butter or rendered bacon fat over low heat, and once the onions were nicely caramelized, add some brown sugar or turbinado sugar to the pan and stir it in, splash in some red and white wines and vinegar to dissolve the sugar, add a little slurry of water and cornstarch to thicken, and let it reduce to a nice glaze, season to taste with salt and pepper, chill. Basically, you're almost making a caramelized red onion chutney, and adding that to the pork. Making a real chutney would be a good alternative, I think.
I'd go for the usual 4-7% or so breadcrumbs, 1.5-1.8% salt, and back into the rest of the spices from there. But that's just me, and off the top of my head. Good luck with your search.
ETA: Tesco's version is a little different, but still similar. 74% pork, 15% red onion. The red onion mixture is red onions, sugar, cider vinegar, cinnamon and cloves. Tesco's uses marjoram and nutmeg, instead of nutmeg, sage and coriander.