Bring a large pot filled with salted water to a boil. Add the pasta and cook until al dente; the timing will depend on the type of pasta used. Drain and toss with 2 tablespoons of the olive oil. Set aside.
Meanwhile, cut off most of the green from the leeks, and then cut the leeks in half lengthwise. Cut the halves crosswise into pieces about ¾ inch wide. Place in a deep bowl of cold water, and let the dirt settle to the bottom. Using a wire skimmer or slotted spoon, transfer to a clean bowl.
Melt 2 tablespoons of the butter with 2 tablespoons of the olive oil in a large saucepan. Add the leeks and sauté, adding a bit of water as needed to moisten, until tender and cooked through, 10 to 15 minutes. Season with salt, pepper, and nutmeg, remove from the heat, and set aside.
Warm 1 tablespoon of the olive oil in a nonstick sauté pan over medium-high heat. Add the sausage and fry until cooked through and golden, about 5 minutes. Using a slotted spoon, transfer to a bowl and set aside.
For the fresh mushrooms, warm the remaining 2 tablespoons butter and 2 tablespoons olive oil in a large sauté pan over high heat. Add the fresh mushrooms and sauté, stirring occasionally, until they give off their liquid, about 6 minutes. Add the cream sauce and the porcini and their soaking liquid, if using. Season with salt, pepper, and nutmeg. Fold in the cooked leeks, the cooked sausage, ½ cup of the Parmesan, and the cooked pasta. Mix well.
Divide among 8 buttered ramekins. Top evenly with the remaining ½ cup Parmesan cheese. Bake until golden and bubbling, about 15 minutes. Serve at once.
Maccheroni con polpette/Pasta with Meatballs: Toss 1 pound cooked pasta with 1 pound batch of Little Pork Meatballs (), 3 cups Tomato Sauce (), ½ pound diced mozzarella, 2 large sautéed diced eggplant, and ½ pound fresh ricotta, topped with ½ cup grated Parmesan. Divide among 8 buttered ramekins. Top evenly with ½ cup Parmesan cheese. Bake until golden and bubbling, about 15 minutes. Serve at once.