Add the first 4 ingredients to the shaker with ice, shake to chill, then strain over ice into a glass. Top with soda. Garnish.
Dew SyrupHeat the Mountain Dew soda syrup (1 1/2 cups of Mountain Dew soda reduced down to 1/2 cup over very low heat, being careful not to boil or scorch) to a near boil over medium heat, being careful not to scald. Reduce the heat and add the remaining ingredients. Break up and smash the lemongrass stalk to release the flavor. Simmer for 10-15 minutes or until the aromatics fully release. Remove from the heat and add lime zest. Allow to steep until cooled to room temperature. Refrigerate for 24 hours and strain. Will last in a refrigerator for up to 2 weeks.