Use a large sharp knife to cut the watermelon in half across the centre. Using the tines of a fork and a toothpick, mark out evenly where to cut a slice for the pizza base. Cut the slice. Repeat with the other half of the watermelon. Lay the 2 slices on a board to create a pizza round.
Cut the watermelon round into wedges to resemble pizza slices.
Drizzle the pizza with the white chocolate and scatter with the pistachio. Serve immediately.
