In medium pot, bring wine, peppercorns, whole garlic, celery, carrots and sliced yellow onion to a boil. Reduce heat and simmer for 5 minutes. Let cool. Place chicken in large bowl and pour cooled wine mixture over the pieces and then drizzle with 1 tablespoon olive oil. Cover and marinate overnight.
Heat oven to 325º and make bouquet garni; tie parsley sprigs, bay leaves and thyme sprigs together, set aside.
Remove chicken from marinade and pat dry. Strain marinade, reserving the solids and liquid separately.
In a large, heavy bottomed pot, heat 1 tablespoon olive oil. Add the bacon and cook until crisp, about 8 minutes. Remove bacon to a bowl with slotted spoon and set aside.
Increase heat to medium high. Working in batches, brown chicken in bacon drippings, 6-8 minutes, and then transfer to a plate. Add the reserved solids and cook until soft, about 10 minutes. Sprinkle with flour and stir and cook for 1 minute. Whisk in reserved liquid and bring to a boil. Stir and simmer for 1 more minute. Stir in remaining garlic, shallots, chicken stock and salt and pepper to taste. Nestle in the chicken and bouquet garni, cover and bake until tender, about 1¼ hours.
When chicken is tender, transfer to a plate and tent with foil. Strain sauce and keep warm on stove.
While chicken is baking, heat 1 tablespoon butter and 1 tablespoon olive oil in a skillet over medium heat. Add peeled pearl onions and cook until lightly browned and golden, about 5 minutes. Reduce heat to low, cover and cook until tender, another 8 minutes or so, stirring occasionally. Combine onions with bacon.
Heat remaining 2 tablespoons butter in skillet over medium high heat. Add mushrooms and cook until tender, about 5 minutes or so. Arrange chicken on a platter, spoon some sauce over and top with bacon, onions, mushrooms and chopped parsley. Seve with mashed potatoes and more sauce on the side. Enjoy.