You might try just stirring in the balsamic vinegar after you've sauteed the onions--I liked the sweetness of the raisins, but the onions with a little tang from the balsamic would be good, too. I added 2 tablespoons of vinegar to one sliced-up onion.

I need to look around for a grocery store that might sell fresh liver, so I can freeze it. Surely somewhere around here must!