Place sliced onions in a mason jar or other non-reactive storage container.
If using glass, make sure it is room temperature or warmer to prevent cracking during pickling.
In a small saucepan combine salt, vinegar, sugar, oregano, cumin, garlic cloves, and bay leaf.
Bring to a simmer over high heat, stirring constantly, until sugar is dissolved and mixture is fragrant.
Pour hot vinegar over the onions.
They should almost immediately begin to turn bright pink.
Cover and refrigerate; they are able to be used within 30 minutes, but they are best if you let them pickle overnight.