Thinly slice 1 large shallot and set it in a small bowl or jar. Bring ¼ cup cider vinegar, 2 tablespoons of sugar, and 1/2 teaspoon salt to a boil in a small saucepan over a medium flame. Simmer for a minute, until the sugar has dissolved, then pour over the hot liquid over the shallot so the slices are fully submerged. Allow to sit for 20 minutes, then place in an airtight container until ready for use. This can be done up to a week in advance of using.