For the tzatziki: Mix together the sour cream, yogurt, lime juice, garlic powder, onion powder, serrano, cucumber, dill and parsley in a bowl with a spoon until well incorporated. Season with salt and pepper. Refrigerate until ready to serve.
For the hummus: Throw the chickpeas, tahini, Dijon, lemon juice, olive oil, salt, oregano and pepper into a food processor and process until smooth.
For the chicken: Preheat the oven to 450 degrees F.
Spatchcock the chicken: Flip the chicken upside down so the breast is on your cutting board. Using kitchen shears, cut along both sides of the backbone to remove it. Season with the za'atar and heavily with salt and pepper.
Heat the olive oil in a large cast-iron skillet over medium-high heat. Sear the chicken skin-side down while gently pressing it until the chicken is a beautiful dark golden brown and the spices are blackened, about 5 minutes. Transfer the chicken skin-side up to a wire rack set on a baking sheet. Roast until the center of the thigh meat reaches 165 degrees F, about 25 minutes.
Serve the chicken on a serving platter with the hummus, tzatziki and warm pita bread on the side.