Roast chicken: Preheat oven to 375°. In a bowl, combine oil, zaatar, salt, and pepper; stir to create a paste. Rub chicken all over with paste, rubbing it into and under skin on breast. Set chicken on a roasting pan coated with cooking spray. Roast until golden brown and crisp, about 1 1/2 hours or until a meat thermometer inserted in thickest part registers 165°. Let stand 15 minutes. Meanwhile, reduce temperature to 350° for croutons. Cut chicken into 8 pieces.**
Make hummus: In a food processor, pulse garlic and rosemary until chopped, about 5 times. Add beans, oil, lemon juice, salt, and pepper. Process until very smooth, adding 2 to 3 tbsp. water if necessary to make a nice dipping consistency. Divide purée among 4 small ramekins.
Make vinaigrette: In a jar, shake all ingredients until emulsified. Shake again just before serving.
Make croutons: In a bowl, toss torn bread with oil; season with salt and pepper. Transfer to a baking sheet and bake until golden brown, 16 minutes, tossing occasionally.
Working quickly, in a large bowl, combine about half of vinaigrette and croutons. Toss to coat, then toss in lettuce. Divide among 4 plates and top each serving with 2 pieces chicken. Serve with ramekins of hummus for dipping chicken pieces. Pass remaining vinaigrette on the side.
*Zaatar usually includes marjoram, oregano, thyme, sesame, and sumac. Find at well-stocked grocery stores and online.
**To cut a roasted chicken into 8 serving pieces, use a good pair of kitchen shears: Cut each breast crosswise into 2 pieces, keeping the wing attached to 1 piece; cut the thighs from the drumsticks, keeping as much meat from the back with the thighs as possible. Discard the backbone or save in the freezer for stock.