Thinly slice the onions using a mandoline, and divide the onions between 2 (16 oz.) jars or 3 (10 oz.) jars.
Place the garlic and peppercorns in each jar.
Heat the vinegar, water, sugar, and salt in a medium saucepan over medium heat.
Stir until the sugar and salt dissolve (about 1 minute).
Let cool and pour over the onions.
Wipe the rim of the jar, and place the flat lid and screw band tightly on the jar.
Process in a water bath for10-20 minutes.
Allow to cool and seal.
Your onions will be ready to eat once they have turned bright pink