To make whipped feta: Combine yogurt, feta, and pepper in a bowl and whip with a whisk until smooth. You can also use a food processor to get a smoother texture.
To compose salad: Place whipped feta in a large salad plate and spread throughout entire surface of the bowl. Place watermelon pieces on top of yogurt dressing. Top watermelon with sumac, mint, toasted sunflower seeds, and a touch of sea salt.