- 3 pounds seedless watermelon, rind removed, cut into 1-inch pieces
- 1/2 cup unseasoned rice vinegar
- 1/4 cup toasted sesame oil
- 3 tablespoons soy sauce
- 2 tablespoons fresh lime juice
- 2 tablespoons tahini
- 1 tablespoon agave nectar
- 1 tablespoon ume plum vinegar (optional)
- Kosher salt
- 2 scallions, white and pale-green parts, thinly sliced
- 1 cup cilantro leaves with tender stems
- Place watermelon in a large resealable plastic bag. Purée rice vinegar, oil, soy sauce, lime juice, tahini, agave nectar, and plum vinegar in a blender until smooth. Pour over watermelon, seal bag, and chill at least 4 hours and up to 1 day.
- Heat a large skillet over medium. Remove watermelon from marinade; set marinade aside. Cook watermelon, tossing often, until lightly caramelized and dried out, 6–8 minutes.
- Meanwhile, bring reserved marinade to a boil in a small saucepan. Reduce heat; simmer until thickened to the consistency of heavy cream, 12–15 minutes.
- Chill watermelon and sauce separately at least 1 hour.
- To serve, toss watermelon with sauce; taste and season with salt as needed. Top with scallions and cilantro.
- Watermelon and sauce can be made 12 hours ahead. Keep chilled.