Slice one stalk of rhubarb lengthwise into slices approximately 1 mm thick.

In a saucepan, combine equal parts sugar and water. Bring to a boil, stirring until the sugar is dissolved.

Once boiling, pour the hot syrup over the sliced rhubarb in a heatproof container. Cover and refrigerate overnight to allow the rhubarb to infuse.

Chop the remaining stalk of rhubarb and place it in a container with the gin. Add a pinch or two of sugar, if desired, to enhance sweetness.

Let the mixture infuse overnight at room temperature to allow the flavors to meld.

The next day, fill a glass with ice.

Pour the infused gin over the ice, followed by the tonic water. Adjust the ratio to your preference; the classic ratio is 1 part gin to 2 parts tonic, but feel free to adjust according to taste.

Add a splash of the simple syrup from the poached rhubarb to sweeten the cocktail slightly, if desired.

Garnish the cocktail with a slice of poached rhubarb.

Enjoy immediately!