Preheat the oven to 400 degrees. Bring a large pot of salted water to a boil.
Zest the lemon and save it in a bowl. Squeeze the juice into another bowl, then whisk in the garlic and melted butter.
Place the salmon, skin side down (if it’s skin on), on a baking sheet lined with aluminum foil. Surround it with the asparagus pieces. Brush the salmon with the lemon garlic mixture, then pour the remaining mixture over the asparagus. Season everything generously with Kosher salt, pepper, and parsley, if desired. Toss to combine.
Bake for 10-12 minutes or until salmon flakes easily with a fork and asparagus is tender. Remove and set aside.
Around the time the salmon goes into the oven, add the pasta to the boiling water and follow package instructions to cook to al dente. Drain, reserving 1 cup of the pasta water in a measuring cup.
While the pasta cooks, make the sauce. Heat the olive oil and butter over medium heat. Add the garlic and reserved lemon zest and saute until fragrant, about 1 minute. Stir in the heavy cream. Bring to a boil over medium high heat, then reduce heat to medium low and continue to simmer until cream is slightly thickened, about 4-5 minutes, stirring often. Stir in the Parmesan cheese and fresh parsley. Taste and season with salt and pepper as needed.
Add the cooked pasta to the cream sauce, stirring to coat. Add pasta water in ¼ cup portions, stirring and allowing the water to blend with the cream sauce and coat the noodles. You may only need 1/4 cup of water. You may need more to get the consistency you want.
Flake the salmon apart, leaving the skin behind, and stir it and the asparagus spears into the pasta. Serve hot with lemon wedges, if desired.