If using fresh mint, wash it and allow to dry completely. It should then be finely chopped. Peel garlic and grate it. Mix garlic and mint well. Peel onions.
Drop a layer of onions in a jar. Pour in the garlic-mint mix and some salt. Repeat until the jar is full. Pour in vinegar to the top of the jar, and place the lid on tightly.
Store in a cool, dry place for 2-3 months. When removing onions from the jar, use a clean, oil-free spoon or fork.