In a measuring cup with pour spout, combine red wine vinegar and water. Add salt and sugar. Stir until completely dissolved.
Cut the red onions using a sharp knife. Remove top and end tips - tunic, basal plate, and roots. Cut in half then slice red onions thinly.
Transfer thinly sliced red onions and black peppercorns in a clean glass container, preferably a glass jar with lid.
Give the red wine vinegar mixture a good stir then pour over red onions. Press red onions, making sure that every slice is fully submerged into the liquid.
Allow the red wine vinegar mixture to completely cool. Tightly cover the glass jar. Refrigerate.
Serve with your favorite meaty mains.
Notes
If preferred, use a mandolin for an even thinly sliced red onions. This Red Wine Vinegar Pickled Onions can be eaten after 30 minutes or an hour at room temperature but it’s best to use them after being stored in the refrigerator overnight for the flavors to mellow.