- 8 cups 3/4-inch seedless watermelon cubes (from 1/2 small watermelon, rind removed), plus more for garnish
- Scant 2 1/4 teaspoons unflavored gelatin (one 1/4-ounce envelope)
- 1/4 cup cold water
- 1/4 teaspoon coarse salt
- 1 1/2 teaspoons fresh lemon juice
- Rose Hip-Hibiscus "Ice Cubes" (http://www.marthastewart.com/258087/rose-hip-hibiscus-ice-cubes), optional
- Mint or basil sprigs, for garnish
- Puree watermelon cubes in a blender or food processor until smooth. Press through a fine sieve into a large bowl and discard solids. Measure 3 cups strained juice.
- Prepare an ice-water bath. Sprinkle gelatin over cold water, and let stand until softened, about 5 minutes. Bring 1/2 cup watermelon juice and salt to a simmer in a small saucepan over medium-high heat. Add softened gelatin, stirring to dissolve. Strain into a bowl set in ice water bath. Stir in remaining 2 1/2 cups watermelon juice and the lemon juice.
- Let stand, stirring often, until just beginning to gel, 15 to 20 minutes. Divide among 4 glasses. Garnish with watermelon cubes. Refrigerate until set, at least 2 hours or up to overnight. Top with rose hip-hibiscus "ice cubes" if desired, and garnish with mint or basil sprigs.
This gelatin sets looser than normal-halfway between a drink and a dessert. Taste the watermelon; if it's not sweet enough, add sugar to the juice.