Melt 2 tablespoons of butter in a skillet. Add the pancetta and sauté until crisp and golden, about 10 minutes. Using a slotted spoon, transfer the pancetta to a large bowl. Melt the remaining butter in the same skillet over medium-high heat. Add the onions, carrots, celery, rosemary and garlic. Sauté until the onions are very tender, about 12 minutes. Gently stir in the walnuts and parsley. Transfer the onion mixture to the large bowl with the pancetta. Add the bread and Parmigiano and toss to coat. Add enough broth to the stuffing mixture to moisten. Season the stuffing to taste with salt and pepper. Mix in the eggs.
Transfer the stuffing to the prepared dish. Cover with buttered foil, buttered side down and bake until the stuffing is heated through, about 30 minutes. Uncover and continue baking until the top is crisp and golden, about 15 minutes longer.