Mix vinegar, sugar and water and the spices in a large pan. Bring it to a boil, and remove from the stove. This is done to dissolve the sugar. Stir a few times
Peel the onions, but leave them whole. Slice them up in thin slices, preferably using a mandolin. Add the onion to the warm brine, and let it all cool off. Store it all in a glass jar in the fridge.