Fill a highball glass with ice cubes and stir them using a bar spoon.
If using, pour 2 dashes of Lemon bitters into the shaker (optional).
Start by measuring and pouring ¾ oz simple syrup into the shaker.
Add 1 oz fresh lemon juice.
Add 2 oz Bourbon to the mix.
Fill the shaker with ice, cap it and give it a good shake (10-15 seconds).
Using the cocktail strainer, strain the mixture from the shaker into the ice filled glass.
Top-up with chilled still or sparkling water and stir gently to combine.
Finally, place a lemon slice and a mint sprig into the glass for garnish.