Slice the red onions very thinly and pack them into a 2 cup mason jar. The slices should look light and flexible, and the jar should feel full without crushing the onions down too hard.

Pour in the 1/4 cup Water and add a generous dash of Kosher salt. If the water is slightly warm, the salt will dissolve more quickly and the brine will look less cloudy after shaking.

Add the 1 3/4 cup Distilled white vinegar until the onions are covered. The onions may rise at first, but they should settle as the liquid moves through the layers.

Seal the jar tightly and shake until the salt disperses through the brine. After a minute or so, the onions will begin to soften and shrink slightly as the vinegar starts working.

Use right away or refrigerate the jar for later. The color becomes brighter pink as the onions rest, and the flavor turns more balanced with a little time.