Preparation

• Split the vanilla bean and scrape out the seeds. Put the vanilla seeds and tea in a small bowl (save the pod for another use). Add the boiling water and steep for 10 minutes. Add the sugar and stir gently to dissolve.

  In a large bowl, combine the watermelon purée, lemon juice, and 1/4 tsp. salt. Strain the tea mixture into the watermelon mixture and stir to combine. Pour into a 9×9-inch metal baking pan, cover with plastic wrap, and freeze. After 1 hour, stir and scrape the mixture with a fork, repeating every 30 to 40 minutes, until the mixture has an icy shard-like consistency, about 3-1/2 hours total.

  To serve, scrape the granita into chilled bowls, and top with a dollop of sweetened whipped cream (if using).