Cut the crusts off the bread and tear remaining loaves into 1-inch pieces; put into a large bowl. Toss with 2/3 cup olive oil. Spread on 2 rimmed baking sheets. Toast on staggered oven shelves in a preheated 400-degree oven 12 to 15 minutes, tossing occasionally, until croutons are golden-brown and crispy on the outside but still a little soft and tender inside. Reverse baking sheets halfway through cooking time. Cool.
Toast the fennel seeds in a small pan over medium heat 2 to 3 minutes, shaking the pan often, until the seeds release their aroma and turn a light golden brown. Coarsely grind the seeds with a mortar and pestle or spice grinder. Set aside.
Heat a large saute pan over high heat for 2 minutes. Add remaining 1/4 cup olive oil and the pancetta. Saute 1 to 2 minutes. Turn heat down to medium, add rosemary sprigs and red-pepper flakes and cook one minute. Add onion, fennel, fennel seeds and thyme. Season with 1/4 teaspoon salt and a few grindings of pepper. Saute about 8 minutes or until the vegetables are lightly caramelized. If making ahead, cool and refrigerate vegetables. Otherwise, add to the croutons and set aside.
Return pan to high heat and pour in white wine. Bring to a boil and reduce by three-quarters. Add chicken stock or broth and bring to a boil. Pour hot liquid over croutons and vegetables, tossing well to combine. Stir occasionally so that the liquid that settles in the bottom of the bowl can be redistributed.
Wipe pan out with paper towels, and return to the stove over medium heat. Swirl in 4 tablespoons butter, and when it foams, add chestnuts. Saute 4 to 5 minutes, until the chestnuts are golden and sizzling. Season with a pinch of salt and pepper; add to the stuffing. Stir to combine, and adjust seasonings if needed. Add eggs and parsley, toss well and put the stuffing into a 2 1/2 -quart casserole. Don't pile it too high — any that remains can be put into a smaller casserole dish. Cover with foil and bake 40 minutes in a preheated 375-degree oven. Remove foil and top stuffing with remaining butter cut into small pieces. Cook uncovered about 15 minutes or until crispy on top.