Tonicto top up with - I used Three Cents Aegean Tonic
Instructions
Find a shaker and put gin, lemon juice, simple syrup, and basil leaves in it.
Also add a good amount of ice cubes.
Shake it well.
Find a lowball glass (or another short glass) with ice and use a fine strainer to pour the contents from the shaker into the glass. If you don't have a fine strainer, just make sure that only the liquid goes into the glass.
Top up the glass with tonic water and optionally lay a couple of fresh basil leaves on top as garnish.