Brew your cold brew using the standard ingredients and procedure. Do this one night before.
Fill your serving glass with some ice cubes.
Pump some simple syrup into the glass.
Pour the tonic water into the glass about three-fourths full.
Measure 2 fl oz of cold brew concentrate.
Slowly pour the cold brew into the drink.
Garnish the beverage with a lemon wheel or orange bitters.
Serve and enjoy your delicious homemade espresso (cold brew) tonic.