1. Preheat the oven to 325°. Lightly butter a 9-by-13-inch glass baking dish. Spread the bread on 2 large, rimmed baking sheets and bake for 30 minutes, stirring twice, until crisp and golden.
2. Meanwhile, in a large, deep skillet, heat 2 tablespoons of the olive oil until shimmering. Add the sausage meat and cook over moderately high heat, breaking it up as you stir, until cooked through, about 10 minutes. Using a slotted spoon, transfer the sausage meat to a large bowl and let cool.
3. Add the remaining 2 tablespoons of olive oil to the skillet and heat until shimmering. Add the pancetta and cook over moderately high heat, stirring occasionally, until softened, about 5 minutes. Add the leeks and chopped celery rib and cook, stirring frequently, until the leeks are softened and golden, 8 to 9 minutes. Spoon off as much fat from the pan as possible. Add the leeks to the sausage meat along with the parsley and celery leaves and let cool.
4. Add the toasted prosciutto bread and all but 2 tablespoons of the Parmesan to the bowl and toss very well. Mix in the stock, season with salt and pepper and spoon the stuffing into the baking dish. Brush with the melted butter. Sprinkle the remaining 2 tablespoons of Parmesan cheese over the top and cover with foil.
5. Preheat the oven to 375°. Bake the stuffing for 45 minutes, or until heated through and lightly browned on the bottom. Remove the foil and bake for about 20 minutes longer, until the top is golden and crisp. Serve hot.
Make Ahead: The recipe can be prepared through Step 4 and refrigerated for up to 2 days. Bring to room temperature before baking.