I agree with the pickled onion vinegar. I use ASDA's pickling onions, peeled, put in a bowl and covered with salt for 24 hours+. Then rinsed, put in jar(s) and filled with Sarson's pickling vinegar. Use after about 3 weeks and use the vinegar as you please. I use a 'dropper' device - a tube with a rubber ball to squeeze and sook up as much as you need.
Cheers,
Andy