In a medium saucepan over medium-low heat, simmer the vinegar with the sugar and ginger, stirring until the sugar is dissolved. Then stir in the rhubarb (with reserved liquid if using frozen).

If you are using fresh rhubarb, simmer it until it is crisp-tender, about 1 minute, then use a slotted spoon to transfer it to a bowl, keeping the liquid behind in the pan. If you are using frozen rhubarb, transfer the rhubarb to a bowl as soon as it returns to a simmer (don’t continue cooking it), also keeping the liquid behind in the pan.

Simmer the liquid until it thickens slightly, about 5 minutes, and then remove the pan from the heat. Stir in the rhubarb. Serve the compote warm or at room temperature.