Make the tomato and watermelon juices separately by juicing fresh fruit and straining through a fine mesh sieve lined with cheesecloth. Chill until ready to use.

Prepare a Collins glass with ice. Slice serrano pepper in half lengthwise, carefully remove and discard ribs and seeds, and add to prepared glass. Measure tomato juice, watermelon juice, vodka, lime juice and salt into a cocktail shaker with a few ice cubes and shake well. Pour drink over ice. Garnish with a skewer of tomato, watermelon, feta and mint.