In a shaker with ice, combine the watermelon juice, vermouth, gin, lime juice, and ginger syrup. Shake until very cold.
- Strain into a coup glass or an oversized wine glass (for extra effect) and garnish with the speared watermelon.
For the watermelon juice:
Remove the rind from ¼ a small watermelon. Reserve the remaining melon for munching on later. Cube the portioned melon and transfer the cubes into a food processor. No need to worry about seeds. Whizz up in the processor until smooth, then strain through a fine-mesh sieve. Discard the pulp, and store the strained juice in the refrigerator until ready to use.