Preheat oven to 350°F. In a small bowl, combine 2 minced cloves garlic, zest of 1 lemon, oregano, 1 tsp salt and ¾ tsp pepper.
In a roasting pan, place chicken, breast side up. Carefully, without tearing the skin, rub garlic mixture under skin. Place remaining cloves garlic, sliced onion, lemon halves and handful parsley into chicken cavity. Roast, uncovered, for 1 hour, 40 minutes to 1 hour, 50 minutes, or until a thermometer inserted in inner thigh, not touching bone, reads 165°F. Remove from oven, tent with foil and let rest for 10 minutes.
Toward end of chicken roasting time, prepare slaw: In a large bowl, combine celeriac, carrots and apple. In a medium bowl, stir together yogurt, avocado, minced onion, cheese, vinegar, maple syrup, lemon zest and juice, and ¼ tsp each salt and pepper. Pour dressing over celeriac mixture and toss well to coat. Stir in chopped parsley and raisins. Serve 1½ cups slaw with 5 oz chicken.
This enchilada-inspired casserole has a mild heat that's balanced by cooling yogurt. If you already have cooked chicken on hand, you can skip the first step - if doing so, you'll only need 2 cups broth.