Place a rack in the lower third of the oven and preheat the oven to 350 degrees F.
Lay the racks of pork on a cutting board with the ribs curving upward. Cut 1/2-inch deep slits in the meat between each rib bone. Stand the racks up back to back, fat sides touching, with the meaty-sides down and the concave curve of the ribs facing upward.
Thread a trussing needle with kitchen string and use it to pierce between the upper third of the two last rib bone ends and tie them together; repeat in the lower third of the last two rib bone ends to tie them together. Repeat on the other end of the racks. Shape the tied meat into an oval, bending where the slits are, to make a crown. Tie a string around the entire meaty bottom of the pork below the other two ties, securing the racks together so they hold their shape.
Transfer the pork to a roasting pan or low-sided braiser. Rub the pork with the olive oil and season with the rosemary, salt and pepper. Fill the center of the roast with Chestnut and Sausage Stuffing, then transfer to the oven.
Roast the pork until the internal temperature in the thickest part of the meat reads 140 degrees F on an instant-read thermometer, about 1 hour 45 minutes,. Transfer to a cutting board or serving platter and let rest for 20 minutes. Pull out all the string ties, then carve into chops and serve with the stuffing.
Chestnut Sausage Stuffing:
Preheat the oven to 350 degrees F. Arrange the bread pieces in a single layer on a baking sheet and toast until lightly golden and dry, about 10 minutes. Transfer to a large mixing bowl.
In a large skillet over medium heat, cook the sausage until browned and no pink remains, about 5 minutes. Add the garlic, celery, fennel and onions to the sausage pan. Add the butter and some salt and pepper. Stir to combine the vegetables and cook until they are softened, 8 to 10 minutes. Next, add the parsley, sage and thyme.
In the mixing bowl with the golden bread chunks, add the chestnuts and chicken broth. Add the sausage mixture and stir to combine. If filling the crown roast of pork, the stuffing is ready to use at this point.
Alternately, butter a 2-quart casserole dish and spoon the stuffing into the dish. Cover the dish with foil and bake until golden brown, 30 minutes.