Make the rhubarb-tea syrup:
In a small saucepan, combine the water, rhubarb and loose tea. Gently heat and keep on a low heat for about 15 minutes, until the rhubarb has softened. Don't let it boil.
Pass the mixture through a fine strainer and chill the liquid. Discard the strained rhubarb and tea pieces.
In a cocktail shaker, add the gin, 1.5 oz of the rhubarb-tea syrup, lemon juice and ice. Shake until chilled and serve with a lemon slice and stick of rhubarb. Top with a splash of citrus soda water, if desired. Enjoy!