How to Make It
In a small saucepan, combine the red wine with the balsamic vinegar, honey, bay leaf, fennel seeds, mustard seeds and crushed red pepper and bring to a boil over moderately high heat. Cover the saucepan and remove from the heat. Let the brine stand for 10 minutes.
In a medium skillet, heat the olive oil. Add the red onion slices, season with salt and cook over moderate heat, stirring the onion occasionally, until the slices are softened, about 6 minutes. Scrape the onion into a bowl.
Strain the brine over the onion and let cool to room temperature. Cover and refrigerate overnight. Bring to room temperature and serve in the brine.