2 T. extra-virgin olive oil
One 3-pound chicken, cut into 8 pieces
salt and freshly ground white pepper
4 T. unsalted butter
12 cippolini onions, peeled and trimmed
4 shallots, peeled and trimmed
2 heads garlic, cloves separated but not peeled
3 sprigs thyme
4 small yukon gold potatoes, peeled and cut into 1 1/2-inch chunks
2 small celery roots, peeled and cut into 1 1/2-inch chunks
2 oz. slab bacon, cut into short, thin strips
12 small cremini or oyster mushrooms, cleaned and trimmed
2 C. homemade unsalted chicken stock or store-bought low-sodium chicken broth
Working over medium-high heat, warm the olive oil in a 12-inch ovenproof sauté pan or skillet – choose one with high-sides and a cover. Season the chicken pieces all over with salt and pepper, slip them into the pan, and cook until they are well browned on all sides, about 10 minutes. Take your time – you want a nice, deep color and you also want to cook the chickens three-quarters through at this point. When the chicken is deeply golden, transfer it to a platter and keep it in a warm place while you work on the vegetables.
Pour off all but 2 tablespoons of the cooking fat from the pan. Lower the heat to medium, add 2 tablespoons of the butter, the onions, shallots, garlic and thyme and cook and stir just until the vegetables start to take on a little color, about 3 minutes. Add the potatoes, celery root, and bacon and cook 1 to 2 minutes, just to start rendering the bacon fat. Cover the pan and cook another 10 minutes, stirring every 2 minutes.
Add the mushrooms, season with salt and pepper and return the chicken to the pan. Cook for 10 minutes, until the vegetables and chicken are completely cooked through. Spoon everything onto a warm serving platter or into an attractive casserole and keep warm while you finish the sauce.
Pour the chicken stock into the pan and bring it to a boil over medium heat, scraping up the bits of vegetable and chicken that may have stuck to the bottom of the pan. Cook the stock at a boil until it is reduced by half. Remove the pan from heat and swirl in the remaining 2 tablespoons of butter.
To serve: Strain the sauce over the chicken and vegetables, and serve immediately with plenty of pieces of crusty baguette to sop up the sauce and spread with the soft, caramely garlic that is easily squeezed out of its skin.