Rinse the porcini mushrooms and place in a small bowl. Add 1 cup hot water, place a small plate or bowl on top of the mushrooms to keep them submerged. Let soak until softened, about 20 minutes. Drain the porcini, reserving the soaking liquid. Chop the porcini.
Meanwhile, heat the oil in a heavy large skillet or Dutch oven over medium heat. Add the onion and cook until beginning to soften, stirring frequently, about 5 minutes. Increase the heat to high. Scrape the sausage from its casing into the skillet. Cook until no longer pink, breaking up the sausage with the back of a fork, about 6 minutes.
Add the fresh mushrooms and rosemary to the skillet. Sprinkle with salt and pepper and sauté until the mushrooms begin to soften, about 5 minutes. Add the porcini and wine. Bring to a boil and boil until almost all the liquid evaporates, about 4 minutes.
Add the broth and porcini soaking liquid to the skillet, discarding any sand at the bottom of the liquid. Simmer until the sauce is syrupy, stirring occasionally, about 25 minutes. (The sauce can be made to this point ahead. Let stand at room temperature up to 1 hour, or cover and refrigerate up to 2 days.)
Cook the pasta in a large pot of boiling salted water until tender but still firm to the bite. I find that the pasta reaches a pleasing consistency in a minute or two less than suggested on the package. Drain well. Return the pasta to the cooking pot.
Meanwhile, reheat the sauce. Add the half and half and boil until thickened somewhat, about 4 minutes.
Add the sauce to the pasta. Add 1½ cups grated Parmesan and mix well to coat the pasta. Season to taste with salt and pepper. Divide the pasta among bowls or plates. Sprinkle with parsley. Serve with additional Parmesan to sprinkle on top.