Start by peeling and thinly slicing the onion, then gently crushing the garlic cloves to help release flavor. Remember, thin onion slices make for faster pickling!
In a saucepan, combine water, vinegar, salt, and sugar, and bring the mixture to a gentle boil. Simmer it until the sugar and salt are dissolved before adding the rest.
Add your onion slices, garlic, and peppercorns to the simmering pickling liquid. Once everything is mixed, turn off the heat, cover your saucepan, and let it sit for about 10 minutes.
Transfer the onions into a mason jar and pour the pickling liquid over them. Remember to tap the jar lightly on the counter to get rid of any air bubbles!