Add 2 ounces vodka (Belvedere or Grey Goose), 1 ounce simple syrup, and 1 ounce freshly squeezed lemon juice to a tall Collins glass.
Fill the glass with ice and top with 4 ounces club soda (Fevertree).
Stir gently, garnish with a lemon slice, and serve.
Notes
Use freshly squeezed lemon juice, not bottled, for the cleanest flavor. To make ahead, stir the vodka, simple syrup, and lemon juice together and refrigerate the base, then add ice and soda water just before serving so it keeps its fizz.
Simple syrup keeps in a sealed jar in the fridge for about a month.