For the simple syrup:

• 1 cup granulated sugar
• 1 cup water

For the cocktail:

• 1 ¹⁄₂ oz. dry, light sherry (such as fino or manzanilla)
• 1 oz. Lillet Blanc
• ¹⁄₂ oz. fresh grapefruit juice
• 3 dashes Angostura bitters
• Soda water
• Grapefruit peel, for garnish

Instructions

1. Make the simple syrup: In a medium pot set over medium heat, stir the sweetener and water to combine. Bring the mixture just to a simmer, lower the heat to low, and cook just until the sweetener dissolves. Remove the pot from the heat and set aside to cool to room temperature. Use immediately, or cover any extras and refrigerate. (Will keep indefinitely.)
2. To make the cocktail, in a shaker filled with ice, combine the sherry, Lillet Blanc, grapefruit juice, ½ ounce prepared simple syrup, and bitters. Cover and shake vigorously until the outside of the shaker feels cold. Strain into an ice-filled Collins glass, top with soda water, and finish with the grapefruit peel.