Step 1

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.

Step 2

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.

Step 3

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.

Step 4

Chill watermelon and sauce separately at least 1 hour.

Step 5

To serve, toss watermelon with sauce; taste and season with salt as needed. Top with scallions and cilantro.

Step 6

Do Ahead: Watermelon and sauce can be made 12 hours ahead. Keep chilled.