Trim the onions; remove the root and some of the peel that comes off easily.
Melt the butter in a large skillet and add the onions. Cook until golden.
Add the Tempranillo and sugar. Simmer until reduced (20 minutes)
Add apple juice and simmer again until reduced to glaze consistency (15 min). Salt and pepper to taste.
Notes
Make this one day ahead! Reheat on low and add a touch more wine or apple juice if needed. Be careful not to overheat or boil or it will become too carmelized and hard when it cools down.
Recipe by Liz's Kitchen at https://www.lizbokisch.com/petite-filet-with-tempranillo-glazed-cipollini-onions