Yes, after your pickled red onions are gone, you can cut up another red onion and put it into the jar for a fresh new batch of pickled red onions. You can also use the leftover vinegar solution to pickle other vegetables like carrots, cucumbers, or cabbage. Another great way to use the leftover vinegar solution is to use it as a tangy salad dressing. Add some plain greek yogurt to the vinegar solution to make a creamy salad dressing. I would only recommend reusing the leftover vinegar solution one more time, as the flavor will get diluted and the vinegar solution will no longer be fresh.