For the Chicken: Whisk together the olive oil, herbs and garlic in a large shallow baking dish. Add the chicken and turn to coat. Cover and let marinate at least 4 hours or overnight.
Preheat oven to 400 degrees. Heat a large saute pan or grill pan over high heat until just smoking. Remove chicken from marinade and season with salt and pepper to taste. Sear each chicken on both sides, and place on a sheet pan. Place in oven and bake for 15 to 20 minutes, until cooked through.
For the Roasted Garlic-Fava Bean Sauce: Place stock and roasted garlic in a medium saucepan and reduce to 3 cups. Strain into a clean saucepan and bring to a simmer. Whisk in the butter and lemon juice and season with salt and pepper to taste. Add the fava beans and cook for 1 minute.
For the Celeriac and Potato Puree: Place the celeriac and potatoes in a large saucepan filled with salt water and cook until soft. Drain, rinse out the pan and run the celeriac and potatoes through a food mill into the pan. Add the butter, warmed milk and beat with a wooden spoon. When the mixture is smooth, season with salt and pepper and fold in the parsley.