Combine 1 cup granulated sugar and 1 cup water in a small saucepan over medium heat. Stir until the sugar fully dissolves and the mixture comes to a gentle simmer.

Once simmering, reduce heat to low and add the lemon zest and lime zest. Simmer gently for 3 minutes to extract oils from the zest, stirring occasionally. Do not boil hard.

Remove the pan from heat and stir in 2 tablespoons fresh lemon juice and 2 tablespoons fresh lime juice. Let the syrup cool to room temperature (about 20–30 minutes) so flavors meld.

Strain the syrup through a fine-mesh sieve into a clean container to remove zest solids. Refrigerate the syrup if not using immediately. This yields roughly 1 to 1 1/4 cups syrup.

Assemble Sierra Mist

Chill serving glasses in the refrigerator for 10–15 minutes, or fill them with ice to chill and discard that ice before serving.

Fill four highball or tall glasses with ice (about 1/2 cup ice per glass). If making a pitcher, use about 2 cups of ice total.

Measure the soda base: combine 2 cups club soda (or sparkling water) and 1 cup carbonated lemon-lime soda in a large pitcher. Stir gently to combine without flattening the carbonation.

Sweeten and brighten: add 3 to 4 tablespoons of the cooled citrus syrup to the pitcher (start with 3 tbsp; adjust to taste). Stir gently. For a sweeter, more commercial Sierra Mist flavor, use up to 4 tbsp.

Taste and adjust: sample the soda. If you want more citrus brightness, add an extra teaspoon of fresh lemon or lime juice. If you want sweeter, add 1/2 tablespoon more syrup at a time.

Pour the finished soda into the prepared glasses, distributing evenly.

Garnish each glass with a lemon slice and a lime slice on the rim. Add a mint sprig if desired for an aromatic lift.

Serve immediately while fizzy. Leftover syrup can be refrigerated for up to 10 days; assembled soda is best consumed immediately and won't hold its carbonation well if stored.