For the raspberry rhubarb syrup: In a medium saucepan over medium-high heat, bring rhubarb, raspberries, and water to a boil. Lower heat to medium-low, cover, and cook until rhubarb is tender, about 10 minutes. Stir to break up fruit and release juices.

Set a fine mesh strainer lined with a double layer of cheesecloth over a large bowl. Pour raspberry rhubarb mixture through strainer; discard any solids. (To get the most juice, pull up the corners of the cheesecloth and gently press on the cheesecloth with the back of a wooden spoon to squeeze out remaining juice.)

Measure the strained juice and add enough water to equal 2 cups. Pour this liquid back into the saucepan over medium-high heat and stir in the sugar. Bring to a boil and cook until sugar dissolves, about 5 minutes. Remove saucepan from heat, stir in lemon juice, and let cool. Strain again through a double layer of cheesecloth, for a clearer syrup, and then pour into a jar. Cap the jar tightly and refrigerate to chill. This can be made 1 or more days prior to serving this cocktail. You will have more syrup than needed for 1 drink.

For the raspberry rhubarb cosmopolitan: To a cocktail shaker filled with ice, add vodka, orange liqueur, 1 ounce prepared raspberry rhubarb syrup, and lime juice. Shake vigorously for 30 seconds to dilute and thoroughly chill. Strain into a martini or other stemmed cocktail glass. Garnish with fresh raspberries and rhubarb if desired.