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.
The pickled onion can be refrigerated for up to 1 week.