Place the sliced onions in a medium-sized, heat proof bowl.
In a small saucepan over medium high heat, mix together the vinegar, water, sugar, and bay leaf. Bring to a boil, stirring, just until the sugar is dissolved, about 1 minute.
Remove from heat. Pour over the onions.
Set aside to cool and marinate for at least 30 minutes. Discard bay leaf.
Onions can be stored in the fridge for several weeks.