These tangy and bright quick pickled red onions will add zip to nearly anything you can think of. A staple in Mexican dining, you'll find that they work especially well with spicy food.
In a small saucepan over medium heat, stir together the vinegar, sugar, cinnamon stick, cloves, bay leaf, star anise, and red pepper flakes. Bring to a boil and then promptly remove the pan from the heat. Cover the pan and let the spices steep for 5 minutes.
Toss the onion slices in the saucepan and toss to coat. Pour the mixture into a 1-quart (960-milliliter) glass jar and cover tightly. Refrigerate for at least 4 hours before serving. The quick pickled red onions will keep for up to a few weeks in the fridge.