1. Make the syrup: Trim the ends of the lemongrass stalk and cut it into ½-inch-long pieces. Smash each piece with the flat side of your knife to release the oils. In a saucepan set over medium heat, combine the lemongrass pieces, sugar, grapefruit peels and ½ cup of water. Bring to a simmer and stir to help the sugar dissolve. Remove from the heat and let cool. Strain through a fine-mesh sieve and store the syrup in a covered container in the refrigerator until needed.

2. Make the cocktail: In a mixing glass filled with ice, combine the Lillet Rosé, sparkling wine and ½ ounce of the lemongrass syrup. Stir until well chilled, then strain into a chilled coupe and garnish with the reserved grapefruit peel.