In a small saucepan, add the vinegar along with 250ml (1 cup) of water. Season with a generous pinch of salt and a few grinds of black pepper. Set over medium heat and bring to a simmer.

Add the onion slices to a large, heatproof jar. Carefully pour the hot vinegar mixture over the onions. Seal the jar and shake to ensure all is evenly incorporated.

Allow the onions to sit in the vinegar until it has cooled to room temperature. Serve then or refrigerate indefinitely. The onions do get better the longer they sit in the vinegar.