Season the chicken with salt and pepper. Rub all over with the za'atar and 1 tablespoon olive oil; set aside.
Toast the pine nuts in a large nonstick skillet over medium heat, stirring, until golden, 3 to 5 minutes. Season with salt and remove to a small bowl.
Return the skillet to medium-high heat and add 1 tablespoon olive oil. Add the chicken and cook until browned on the bottom, about 5 minutes. Flip, reduce the heat to medium and continue cooking until the chicken is cooked through, 8 to 10 more minutes. Transfer to a cutting board and let rest.
Meanwhile, halve the cucumbers lengthwise and slice 1/2 inch thick. Combine in a large bowl with the watermelon, remaining 2 tablespoons olive oil, the lemon juice and mint; season with salt and pepper and toss.
Stir the hummus and yogurt in a small bowl; spoon onto plates. Slice the chicken and place on top of the hummus. Drain off any excess liquid from the cucumber-watermelon salad and add to the plates. Sprinkle with the pine nuts and more za'atar.