Peel the red onions and cut each in half. Cut each half into quarters. If the onions are large, cut them into bite-sized pieces instead, avoiding pieces that are too small as they will overcook.
Heat 2 tablespoons olive oil in a large pan over medium heat. Add the onions and 5 sage leaves, and cook for 1-2 minutes.
Pour in the 100 ml balsamic vinegar and 50 ml red wine, stirring to deglaze the pan.
Stir in 2 teaspoons honey. Reduce heat to medium-low and simmer for 5 minutes.
Season with salt, pepper, and peperoncini to taste.
Transfer to a clean container and refrigerate for at least 4 hours, preferably 24 hours, for the flavors to develop fully.