Make the rosemary simple syrup. Combine sugar and water in a small saucepan over medium heat. Stir until the sugar dissolves, then add the rosemary sprigs. Simmer for 1–2 minutes, remove from heat, and let steep for 15–20 minutes. Strain and cool. Store covered in the fridge for up to two weeks.

Juice your citrus. Squeeze fresh grapefruit and lemon. Strain out pulp if you prefer a cleaner texture. Fresh juice makes a big difference here.

Fill a Collins glass with ice. A tall glass helps show off the bubbles and aroma. If you want it extra cold, chill the glass ahead of time.

Shake the base. In a cocktail shaker filled with ice, add 2 ounces gin (or vodka), 2 ounces grapefruit juice, 1/2 ounce lemon juice, and 1/2–3/4 ounce rosemary simple syrup. Shake briskly for 10–12 seconds to chill and blend.

Strain and top. Strain into the ice-filled Collins glass. Top with 2–3 ounces of club soda, depending on how strong you like your drink.

Garnish. Smack a rosemary sprig between your hands to release its oils, then tuck it into the glass. Add a grapefruit wedge or a thin wheel on the rim.

Taste and adjust. If it’s too tart, add a tiny splash more syrup. If it’s too sweet, squeeze in a touch more lemon or add more soda.