Butter a large 33 x 23-cm (13 x 9-inch) baking dish or with a capacity of at least 3 litres (12 cups).
In a large and deep non-stick skillet or a large saucepan, sauté the squash, onions, celery, apples, and garlic in half the oil for about 10 minutes. Season with salt and pepper. Transfer to a large bowl.
In the same skillet, brown the pancetta and ground meat in the remaining oil. Add the mustard. Season with salt and pepper. Spoon over the vegetables and add the remaining ingredients. Stir to combine and spoon into the baking dish. Bake the stuffing in the oven during the last 30 minutes of cooking the turkey, on the bottom rack. While the turkey is resting, increase the oven’s temperature to 180 °C (350 °F) and bake the stuffing for another 30 minutes or until it is firm and golden brown. Serve with the Roasted Turkey with Sage Butter (see recipe).
If you do not roast a turkey, bake the stuffing in a preheated oven at 180 °C (350 °F) for about 1 hour or until the it is firm and golden brown.