To begin, prep all of your ingredients. If you don't have simple syrup already, make it by adding 1/2 cup water and 1/2 cup granulated sugar to a small saucepot, and bringing to a simmer. Once the liquid is simmering, reduce heat and stir frequently until the sugar is totally dissolved. When done, set aside to cool. (This will make more syrup than you will need for the cocktail - save the rest for more cocktails in the future!)
Wash and dry your nectarine. First, cut off 2-4 slices of the fruit for garnish later on, and set aside.
Then, chop the remaining nectarine into rough pieces that are small enough to fit in your cocktail shaker. Add the pieces to the shaker with the lemon juice. Using a muddler or a heavy spoon, smash the nectarine until it takes on the texture of applesauce.
Then, add in the rhubarb gin and simple syrup, plus 3-4 large ice cubes. Shake vigorously for at least 30-45 seconds.
Take two cocktail glasses of your choosing and fill them to the top with ice. Pour the mixture into the glasses, evenly distributing the drink between the two.
Garnish with your reserved nectarine slices, and enjoy!