In a small saucepan, bring the vinegar, water, agave, and salt to a simmer then remove from heat.
Slice the red onion thin along the natural lines of the exterior, then place the slices in a jar. and add in any additional flavorings that you like (I use what I have on hand but you can also go without).
Pour warm vinegar mixture over the onions and let sit at room temp for 20-30 minutes. Cover the jar with a tight lid and store in the fridge for 1-2 weeks.
Serve with tacos, over salads, over potato pancakes, etc etc.