Shred carrots. Slice onions across, then chop the rings so they break apart into lengths. Stuff into Mason jar.
In a small saucepot, simmer together 2 cups vinegar (apple cider, white, distilled, red… balsamic is a step too far and I do not recommend), 1 cup (? maybe less?) sugar, spices (pickling or some combination of mustard seeds, celery seeds, peppercorns, float your own boat). Cover the vegetables in the jar with the hot syrup. Top, allow to cool on counter, refrigerate.