Combine feta and yogurt in a large bowl and whisk until well blended. Add lemon zest and juice, olive oil, 1 1/2 teaspoons dill and 1 1/2 teaspoons mint. Season to taste with salt and pepper. Mix until well blended. If not serving immediately, cover and chill until ready to serve.
Transfer to a serving bowl and garnish with the remaining herbs, a drizzle of olive oil and a sprinkling of sesame seeds. Serve with watermelon spears.