Fill a highball glass or plastic cup with ice. Pour limoncello or lemon syrup over ice, then stir in club soda. Add a squeeze of lemon juice. Adjust flavors to your liking with more limoncello, syrup, club soda, or fresh lemon.
*Brands of limoncello and lemon syrup vary in sweetness. We tested the recipe with two Western brands: Ventura Limoncello Company's excellent Originale (venturalimoncello.com) and Meyer Lemon Infused Simple Syrup from Sonoma Syrup Co. (sonomasyrup.com).
Note: Nutritional analysis is per serving.