I’ve had watermelon and feta salads before but never one this refreshing and unusual. My friend Julie Logue-Riordan, a Napa Valley cooking teacher, brought it to a potluck at my house and it was the hit of the evening—cool and crunchy, sweet and minty. With tomatoes and watermelon both at peak flavor, now’s the moment. If you can get tomatoes in multiple colors—well, wow.