This is such a refreshing salad you can prep ahead of time for a dinner or BBQ!
Combine the lime juice, scallion whites, and serranos in a large bowl and let sit for 5 minutes. Stir in the sugar, if using, and salt.
Add watermelon, ½ cup cojita cheese, ¼ cup cilantro, ¼ cup pepitas, and the scallion greens. Stir to combine.
Transfer to a shallow serving dish. Sprinkle the remaining ¼ cup cojita cheese, remaining 1 tablespoon cilantro, and remaining 1 tablespoon pepitas and serve.
You can make the salad ahead of time, by cutting the watermelon the night before and refrigerate overnight. Make your dressing also ahead of time.