Adjust rack to lower part of oven. Preheat oven to 425° (220C). Remove fat from chicken and put in a medium saucepan over low heat to until fat renders out. Wipe out cavity with a paper towel and season chicken inside and out with salt and pepper. Remove skin flap from neck and chop into 1-inch pieces.
In a small bowl, combine oil, garlic, ¼ tsp salt and pepper, and thyme. Mix and then brush each bread slice with mix. Rip each slice into 3 and stuff into chicken cavity. Truss the chicken loosely with string, pat dry then cover the breast with a the pancetta slices with string.
Put 2 tbsp rendered chicken fat or, if prefer, 2½ tbsp of unsalted butter in the saucepan over low heat. Add onion, carrot, and leek. Cover and cook until softened, about 5 minutes. Stir in chopped neck bones and cook a minute more.
Roll the trussed chicken in the vegetable mix. Place chicken in casserole, back-side up. Toss some of the vegetables over the top. Transfer to the oven and roast, uncovered, for 15 minutes.
Lower oven to 350° (177°C), turn chicken on its side, cover and roast for another 20 minutes. Turn it to its other side, and roast another 20 minutes. Turn chicken breast side up and remove pancetta. Put back in oven, this time uncovered, for another 20 minutes until golden brown. Baste occasionally with pan juices. When done, juices will run clear when thickest part of thigh pierced or a meat thermometer reads 160°.
Remove chicken to a cutting board and tent loosely with tin foil. Strain the cooking juices into a bowl, pressing down on the vegetables and neck bones to extract all their flavor. Deglaze the pot with 1 cup of water and bring to a boil, scraping up the brown bits on bottom and sides of the pan. Separate as much of the brown juice from the fat in the bowl and pour the juice into the pot. Boil down to a little over 1 cup. Taste and adjust seasoning as necessary.
Untie the chicken and spoon the stuffing onto a serving dish. Saturate the stuffing with half the sauce. Carve the chicken and arrange over stuffing. Serve with pomme frites or mashed potatoes and a green vegetable with remaining sauce on the side.