Sprinkle the watermelon rind with the salt and toss to combine. Set aside for 30 minutes until the moisture has drawn out. Drain the rind and pat dry on a paper-towel-lined baking sheet.
Add 1 cup of the watermelon flesh, fish sauce, sugar, garlic, ginger and onion to a food processor. Pulse until combined. Transfer to a bowl and stir in the Korean chili flakes. Add the drained watermelon rind and the scallions and toss to combine. Transfer to a jar and let it rest for at least 12 hours before serving.