Bring a large pot of salted water to a boil. Meanwhile, in a large, high-sided frying pan over medium heat, melt butter. Add onion and cook, stirring, until onion is soft but not browned, about 3 minutes. Stir in cream, lemon zest and juice, 1/2 tsp. salt, and 1/2 tsp. pepper. Reduce heat to medium-low and cook until mixture has thickened slightly, 4 to 5 minutes.
Add pasta to water and cook according to package instructions. Drain, reserving 1 cup cooking water.
Add salmon to cream mixture and stir to combine. Pour pasta into frying pan with salmon-cream mixture and toss to coat, adding some of the cooking water as needed to moisten pasta. Add parsley and toss to combine. Season to taste with salt and pepper.
Note: Nutritional analysis is per serving.