6 packed cups seedless watermelon cubes (precut is fine)
1/2 cup/100 grams sugar, more to taste
2 tablespoons lemon juice (from 1 lemon)
1/2 cup/75 grams cornstarch
1/4 cup/32 grams chopped pistachios
Mint leaves and jasmine or other fragrant blossoms, for garnish (optional)