Prepare the Onions: First, peel the red onions. Slice them in half from root to tip, then slice them thinly into half-moons. Aim for slices that are about 3-5mm thick. What works best for me is using a mandoline slicer for perfectly uniform slices, but a sharp chef's knife and a steady hand will work just as well.

Pack the Jar: Tightly pack the sliced onions into a clean, sterilised 500ml glass jar. If you're using any of the optional flavourings like peppercorns, garlic, or a bay leaf, add them to the jar now, scattering them amongst the onion slices.

Make the Brine: In a small saucepan, combine the white wine vinegar, water, caster sugar, and sea salt. Place the pan over a medium heat.

Heat Gently: Warm the brine mixture, stirring occasionally, for about 3-5 minutes. You are not trying to boil it vigorously; you just want to heat it enough for the sugar and salt to dissolve completely. Once you can no longer see any granules, remove it from the heat.

Combine and Pickle: Carefully pour the hot brine directly over the onions in the jar, ensuring they are fully submerged. You'll almost immediately see the onions begin to turn a beautiful, vibrant pink.

Cool and Steep: Use a spoon to gently press the onions down to remove any trapped air bubbles. Screw the lid on the jar and let it cool on the countertop for about 30-60 minutes. Once cooled, they are ready to serve, or you can transfer the jar to the fridge for storage.