1 1/2 pounds lean ground lamb
Preheat the oven to 350 degrees F. Heat a large skillet over medium-high heat. Add the lamb and cook, stirring to break up the meat, for 2 minutes. Remove from the heat and drain in a colander.
Discard the fat from the pan, return the meat to the pan, and cook, stirring, for 3 minutes. Add the onions and cook, stirring, for 3 minutes.Add the garlic and cook, stirring, for 30 seconds. Add the prosciutto and tomato paste, stir well, and cook for 1 minute. Stir in the pine nuts. Add the tomatoes and their juices and bring to a boil.
Reduce the heat to medium-low and simmer, stirring occasionally, for 25 minutes. Add the oregano, rosemary, salt, and pepper, and stir well. Remove from the heat and let cool.
Repeat with the remaining ingredients, for 3 layers in all. Sprinkle the top with the remaining 1/4 cup Parmesan.Cover tightly with aluminum foil and bake for 50 minutes. Uncover and bake until bubbling and golden brown, about 15 minutes longer. Remove from the oven and let sit for 15 minutes before serving.