In a medium bowl, toss 2 pounds of salmon (cut into 1-inch pieces) with 2 teaspoons of Cajun seasoning and 1 tablespoon of olive oil until evenly coated.
Bring a large pot of salted water to a boil. The water should be well salted (see notes).
Heat the remaining 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the salmon in a single layer and sear for 2–3 minutes per side, or until golden and cooked through (see notes). Transfer to a clean plate, tent with foil, and set aside.
Once the water is boiling, add 10 ounces of fettuccini noodles and cook according to package directions until al dente. Before draining, reserve 2½ cups of pasta water, then drain the noodles and set aside.
In the same skillet that was used to cook the salmon, melt 4 tablespoons of salted butter over medium heat. Add 2 diced shallots and cook for 3–4 minutes, or until translucent.
Stir in 3 cloves of minced garlic, ½ teaspoon of salt, and ½ teaspoon of black pepper. Cook for about 30 seconds, just until fragrant.
Pour in the reserved 1½ cups of pasta water, scraping up any browned bits from the bottom of the skillet. Simmer gently for 2–3 minutes.
Add the cooked pasta and 1 cup of heavy cream to the skillet. Toss vigorously for 1–2 minutes to combine and emulsify the sauce.
Remove from heat.
Stir in 1 cup of freshly shredded mozzarella cheese and ½ cup of freshly grated parmesan cheese until the sauce is smooth and creamy.
Add additional reserved pasta water, if needed, to thin the sauce, then season with additional salt and black pepper, to taste.
Return the cooked salmon pieces back to the skillet or serve the creamy pasta topped with desired amount of seared salmon pieces. Garnish with freshly chopped parsley, if desired. Enjoy!