In a large skillet, over medium heat, sauté the onion in the olive oil until golden. Add the sausage, breaking it apart with a fork until crumbled. Cook the mixture until the sausage is cooked through. Add the walnuts, salt and pepper to taste.
Meanwhile, in a large pot of salted water, cook the pasta until al dente. Drain the pasta, retaining one cup of the cooking water. Add the pasta to the skillet with the sausage, mixing thoroughly over medium heat. Remove from heat and add in the cheese until evenly distributed. Add a bit of the retained cooking water if it seems dry. Serve immediately with a sprinkle of Parmigiano.