Place the sliced onion, garlic cloves, dill, and peppercorn in a clean pint jar. Depending on the size of your onion, it might not all fit. You don’t want to overpack the jar but feel free to stuff it pretty well.
In a small saucepan, combine the water, red wine vinegar, and salt. Heat until just simmering, stirring to dissolve the salt.
Carefully pour the hot vinegar mixture over your vegetables. Give the vegetables a quick stir to make sure there aren’t any air pockets hiding. Discard any leftover vinegar mixture. On the other hand, if you don’t quite have enough vinegar mixture to cover the vegetables, top the jar off with more vinegar.
Cover the jar with a plastic lid or place a square of wax paper between the jar and a metal lid so the vinegar doesn’t corrode the metal. Let the pickles cool to room temperature before placing them in the fridge.
Once chilled, the pickles are ready to use! They’ll last in the fridge for a few weeks and continue to develop their flavor as they sit.