Chilled Watermelon Soup

Makes 4 servings.
1-inch piece ginger, peeled and finely chopped
1 tbsp brown sugar
2 limes: one for zest and both for juice
3 cups cubed and seeded watermelon
2 mint leaves, plus more for garnish
1/4 cup sparkling wine (or sparkling water)
In a small nonstick skillet, gently sauté ginger, sugar, and zest over medium heat 2 minutes. Add lime juice and simmer until no liquid remains. Transfer mixture to a blender and add watermelon, mint, and sparkling wine. Puree until smooth; strain with sieve into bowl. Chill soup at least 3 hours before serving. For a more elegant version: Divide soup into 2 equal portions. Freeze a portion for 3 hours to make a granité (a slushy of sorts) and chill remaining half for 3 hours. Scrape granité into 4 bowls and pour chilled soup over granité. Garnish with mint leaves; serve immediately.