How to Make It
Bring agave nectar to a boil in a small saucepan over medium-high. Add lemongrass, and remove pan from heat. Cover and let stand at room temperature 8 hours or overnight. Remove lemongrass, and discard.
Stir together lemongrass syrup, tequila, grapefruit juice, lime juice, and seltzer until blended. Serve over ice in salt-rimmed glasses with lime wedges.