How to Make It
In a medium bowl, whisk the grapefruit juice with the sugar and brandy until the sugar is dissolved. Chill the grapefruit cordial for 15 minutes.
In a large measuring cup, stir the fennel juice with 2 ounces of the grapefruit cordial. Pour the drink into 4 tall ice-filled glasses and top with the cider. Garnish with fennel fronds and serve.