Brew a double espresso shot with Moka pot and allow it to cool (You can use a coffee maker as well).
Pour espresso shots into a rocks glass over ice cubes, then pour in 150 ml of tonic water.
Cut the lemon and squeeze half of the lemon into the glass.
With a spoon give it a stir.
Add 1 tsp Ginger syrup on top for extra flavor (Optional.)
Notes
If you don't have access to espresso, cold brew will also suffice. Experiment with the cold brew to tonic water ratio, pouring most or all of it as you like.