Slice the red onion thin and place it in a small bowl. Cover with cold water and let it soak for ten minutes. This takes away the sharp raw bite without washing out the color or crunch. Drain and pat the onion dry before using.

While the onion soaks, slice the cucumbers into rounds about one quarter inch thick. A mandoline makes this fast and even, but a sharp knife works perfectly well. Toss the slices with kosher salt in a large bowl and let them sit for ten minutes to release some of their water. Drain off any liquid that collects.

Make the brine. In a separate bowl or large measuring cup, whisk together the white wine vinegar, water, sugar, sliced garlic, black pepper, and red pepper flakes if using. Stir until the sugar dissolves completely.

Add the drained cucumber slices, soaked red onion, and fresh dill to the brine. Stir gently so everything is coated. Taste the brine at this point. If you want it sharper, add a small splash more vinegar. If it tastes too tart, add a pinch more sugar.

Pack everything into a glass jar or airtight container. Press the cucumbers and onion down so they are mostly submerged in the brine. Seal and refrigerate for at least four hours. Overnight is better, especially for the garlic flavor to really come through.

Serve cold, straight from the jar with a spoon, or transfer to a bowl and scatter a few extra sprigs of fresh dill on top. Offer some of the brine alongside so people can spoon it over grains or use it as a dipping drizzle.