A shrub isn't just a low-lying bush. It's a method of preserving fruit that stems from the colonial era: combining sugar, fruit and vinegar to make a well-balanced syrup. These syrups are being added to modern cocktails to add a sour tang that balances the warmth of the alcohol nicely.  This winter appropriate shrub is tart and tangy, with just enough sweetness to balance it and add an intriguing sour note to any cocktail.  It is easy to make at home too.  Try it with a vodka or even gin.