Transfer the vinegar to a small saucepan and bring it to a gentle simmer over medium-high heat.
Once the vinegar is simmering, remove the saucepan from the heat and add the cloves, star anise, ginger, cinnamon, seeds, black peppercorns, and agave if using. Set aside for 10 minutes.
Meanwhile, slice the red onion with the grain using a sharp chef’s knife or mandolin.
If using red chili pepper, slice roughly and remove the seeds, or include the seeds for more spice.
Place the onion and red chili into a glass container with a tight-fitting lid. Pour the vinegar and spices over the onion.
Add 1 cup water, plus more as needed to cover the onion.
Refrigerate. These No-Cook Pickled Red Onions can be used immediately but taste best after being refrigerated overnight.
Notes
To decrease the sharpness of red onions, onion into thin slices and cover them with boiling water. Set aside covered for about 30 minutes. Drain.
For more flavor let the spices simmer in the vinegar for 5 minutes. Yet, refrigerating the vinegar allows the flavors to concentrate further over time.
Enjoy the vinegar in dressings and consume the spices with the onions!
Storage: Refrigerate Quick Pickled Onions (no sugar) in an airtight container for up to 7 days. I used this container and it works very well as you can flip it over and easily remove the onion. (This is a paid affiliate link but not sponsored.)