Wrap the rhubarb and the cucumber slices around the inside of a double old fashioned glass. Layer in ice cubes, fruit and mint leaves. Pour the Pimm's No.1 and the lemonade over the fruit and ice and stir gently to combine. Fill to the top of the glass with ginger beer then garnish with a mint sprig and a cherry if desired. Serve immediately.
To make a small pitcher of this cocktail simply quadruple the amount of ingredients called for above. 8-10 hours prior to assembling the pitcher freeze lemonade in ice cube molds. Then follow the instructions above layering the fruit and lemonade ice cubes into a serving pitcher then pour in the Pimm's, lemonade, and 3-4 ounces of ginger beer. Stir gently and serve.