Thinly slice the onions (it's helpful to use a mandoline), and divide the onions between 2 (16-ounce) jars or 3 (10-ounce) jars. Place the garlic and peppercorns in each jar, if using.

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. Set aside to cool to room temperature, then store the onions in the fridge.

Your pickled onions will be ready to eat once they're bright pink and tender - about 1 hour for very thinly sliced onions, or overnight for thicker sliced onions. They will keep in the fridge for up to 2 weeks.