Pour ingredients into a cocktail shaker over ice, shake, strain into a coupe, then garnish with a citrus twist.
Combine the rhubarb, apple cider vinegar and sugar in a saucepan over medium heat, stirring as rhubarb begins to break down.
When it comes to a boil, reduce heat to low and cook until rhubarb is completely broken down, about 10 minutes, stirring occasionally.
Remove from heat, pour into a colander or fine mesh strainer, and discard the solids. Store shrub in the refrigerator.