Add the tomatoes, watermelon and mint to a small bowl
Mix the sumac, vinegar, olive oil and salt in a small bowl and drizzle over the salad
Mix well, making sure not to break up the tomatoes or watermelon. Serve within a few hours of making the salad - either cold or room temp (the watermelon is best served cold so I like to keep it in my fridge until I'm ready to assemble the salad)