Linguine with Leeks, Prosciutto & Lemon

Bring a large pot of salted water to a boil for cooking the pasta. Chop the trimmed prosciutto fat, if using, and put it in a large skillet with the butter. Heat over medium until the butter is melted and the fat is translucent. Add the leeks, season with salt, pepper, and nutmeg, and cover. Cook, stirring occasionally, until tender but not browned, about 10 minutes. Add the prosciutto, stirring to distribute, and then add the cream. Reduce the heat to low and let heat gently, uncovered, while you cook the pasta until just tender. Reserve 1 cup of the pasta cooking water and then drain the pasta, leaving drops of water clinging to it. Add the pasta and half of the reserved pasta cooking water to the skillet. Stir in the lemon zest and juice; season abundantly with black pepper. Heat gently for a few minutes to let the pasta drink up the sauce, adding more pasta cooking water if needed to thin it. Add the walnuts and taste for salt and pepper. Serve immediately.