For ginger-pepper dressing, dry-roast spices in a small frying pan over medium heat until fragrant (30-40 seconds; see note), then crush with a mortar and pestle. Combine with remaining ingredients and a large pinch of salt in a jar and shake to combine. Season to taste and set aside.
Combine watermelon, herbs, spring onion and pickled ginger in a bowl and toss gently. Transfer to a platter, drizzle with ginger-pepper dressing and serve.