This fizzy spring forward cocktail uses a rhubarb saffron shrub—shrubs are a form of rinking vinegar made with fresh fruits—since rhubarb is starting to come into season and has a great tartness to it, let it sit in some sugar, with its natural juices diluting the sugar. Adding some saffron adds a great depth of flavour. A good red wine vinegar highlights some great acidity to a twist on a classic Tom Collins.