Muddle the rhubarb and rosemary - In a shaker, muddle the diced rhubarb and one sprig of rosemary until the rhubarb is crushed and the rosemary is aromatic. Add ice cubes to the shaker. - If you don't have a cocktail shaker, you can use a large mason jar with a tight lid instead. Be sure to crush the rhubarb well to release its juice.

Add gin, lemon juice, and simple syrup - Add 3 oz of gin, 1/2 oz of lemon juice, and 1 oz of simple syrup to the shaker. - To make the cocktail sweeter, you can add more simple syrup. If you prefer your drink less sweet, adjust the amount of lemon juice and simple syrup to your liking.

Shake and strain into glasses - Shake the cocktail shaker vigorously for 10-15 seconds until the ingredients are well mixed. Strain the cocktail into two glasses, preferably highball or Collins glasses, filled with ice cubes. - To make the drink even more refreshing, you can add some slices of fresh cucumber to the glasses before pouring the cocktail.

Top with soda water - Pour 1 oz of soda water into each glass and stir gently to combine. Add a sprig of fresh rosemary and enjoy!