Chill your martini glass by filling it with ice water and letting it sit for 5–10 minutes.

In a cocktail shaker, add 2 ounces of fresh lemon juice and 1 ounce of simple syrup. Fill the shaker with ice and secure the lid.

Shake the cocktail shaker vigorously for about 15 seconds until cold.

Discard the ice water from your chilled martini glass and strain the lemon mixture into the glass.

Gently pour in 3–4 ounces of chilled club soda or sparkling water, pouring slowly to maintain fizz.

Add a lemon twist or slice as a garnish and serve immediately.