Stir the first 4 ingredients in a large measuring cup.
Place cucumber slices around the inside of a frosty cocktail glass, about 4 per glass.
Pour liquid mixture into the glasses.
Garnish with an extra slice of cucumber, if desired.
Notes
To make frosty glasses, wet inside and outside of cocktail glasses and pour water out. Place wet glasses in the freezer for 30 minutes to an hour. This step is absolutely optional. I just like how much cooler the drink feels in my hand. DO NOT SHAKE THIS COCKTAIL. Tonic water will explode or fizz everywhere like shaking a can of coke. Stirring is preferred.To make the sage simple syrup, take 6 large sage leaves (or 12 small) in the palm of your hand and slap them with your other hand. This releases the oils and makes the infusion even better. In a small saucepan, add all of the sage, 1/2 cup sugar, and 1 cup water. Whisk it together and cook until it comes to a boil and the sugar dissolves, about 3 minutes. Remove from heat and cool for 30 minutes. Either strain out or remove sage leaves.If you need fine mesh strainer, click here for the set I use and highly recommend.