2 tablespoons simple syrup* (plus more)
Sugar-in-the-raw for the rim of the glass
1/2 lime, quartered
10 whole leaves fresh mint
2 ounces rum
6 to 8 pieces of ice
1/2 cup club soda (plus more)
Dip a finger in the simple syrup and run it along the rim of a highball glass. Dip the rim in sugar-in-the-raw to coat. Place 3 pieces of lime and all of the mint leaves in the bottom of the highball glass and press the side of a spoon against the side of the glass to crush the lime wedges and mint (to release the oils from the mint and juice the limes). This is called muddling, and you can use a muddler if you have one.
Add the rum, and stir to combine. Fill the glass with ice, and then with club soda. Add the final lime wedge, then stir and serve immediately.
*To make simple syrup, heat equal parts water and sugar-in-the-raw in a small saucepan over medium heat just until the sugar is completely dissolved (about 1 minute). Remove from the heat and allow to cool completely before using. Store, covered, in the refrigerator for up to 2 weeks.