Put 2 or 3 cups of water on to boil in a kettle. Peel and thinly slice the onions into approximately 1/4" moons. Peel and cut the garlic clove in half, length-wise.
In the jar you will be using to store the onions, add the sugar, salt, vinegar and flavorings. Stir to dissolve.
Place the onions in a heat-proof glass bowl and pour boiling water to cover. Let sit for 30 seconds, then carefully strain the onions.
Add the softened onions to the jar and stir to evenly distribute the flavorings.
The onions will be ready in about 30 minutes, but are best after a few hours. Store in the refrigerator. They will keep for several weeks but are best in the first week.