Make the rosemary simple syrup: Combine 1/2 cup sugar and 1/2 cup water in a small saucepan.
Add 2 rosemary sprigs. Bring to a gentle simmer, stir to dissolve, and remove from heat. Let steep 15–20 minutes, then strain and cool.
Store in the fridge.
Salt the rim (optional): Run a lime wedge around half the rim of a highball or rocks glass. Dip that side into kosher or flaky salt. Salting only half gives control with each sip.
Fill the glass with ice: Use fresh, solid cubes.
More ice keeps the drink colder and prevents dilution.
Shake the base: In a shaker, add 2 oz blanco tequila, 2 oz fresh grapefruit juice, 0.5 oz fresh lime juice, and 0.5–0.75 oz rosemary simple syrup (to taste). Add ice and shake 10–12 seconds, until chilled.
Strain and top: Strain into the prepared glass over fresh ice. Top with 2–3 oz club soda or sparkling water.
Give a gentle stir to combine.
Garnish: Smack a rosemary sprig between your palms to release oils and place it in the glass. Add a grapefruit wedge or thin slice on the rim.
Taste and adjust: If it’s too tart, add a dash more syrup. If it’s too sweet, squeeze in a touch more lime or add extra soda.