Shake all ingredients except soda water in shaker until ice cold. Add soda water and swirl to mix. Strain into 2 glasses. Use peeler to shave ribbons off rhubarb stalk for the garnish.
For the rhubarb syrup:
Place all ingredients in a saucepan and bring to a boil. Stir until sugar has dissolved, then reduce heat to simmer for 20 minutes. Strain through fine mesh strainer and press all the liquid out. Allow to cool before storing in an airtight container. Will last refrigerated for up to 2 weeks.