In a large frying pan, melt the butter. Add the onion, lemon zest, cream, shrimp, and salt to taste. Bring to a boil. Immediately reduce the heat, and simmer for 10 minutes, stirring occasionally.

Meanwhile, bring 4 quarts of water to a boil. Add 1½ tablespoons of salt. Stir in the pasta and cook until al dente, stirring often. Drain, reserving 1 cup of hot pasta water.

Add the pappardelle to the pan with the sauce and toss well. Add a little hot pasta water if too dry. Cook over medium-low heat for 3 or 4 minutes. Sprinkle with pepper and parsley and toss well. Serve immediately. Pass the Parmesan cheese.