Cook the fettuccine pasta according to package instructions in a large pot of salted water until al dente. Drain and set aside.
While the pasta is cooking, prepare the salmon. Season both sides of the salmon fillets with Cajun seasoning, ensuring they are evenly coated.
In a large skillet, heat the olive oil over medium heat. Once hot, add the seasoned salmon fillets skin-side down. Cook for about 4-5 minutes on each side until the salmon is cooked through and flakes easily with a fork. Remove from the skillet and set aside.
In the same skillet, add the chopped onion and sauté for 2-3 minutes until translucent. Then, add the minced garlic and sauté for an additional minute until fragrant.
Pour in the heavy cream, stirring to combine. Allow it to simmer gently for about 2-3 minutes until it thickens slightly.
Add the cherry tomatoes and spinach to the skillet, cooking until the spinach wilts and the tomatoes soften, about 2-3 minutes.
Flake the cooked salmon into large chunks and gently fold it into the creamy sauce. Season with salt and pepper to taste.
Toss the cooked fettuccine into the skillet with the sauce and salmon, ensuring everything is well combined and coated in the creamy Cajun sauce.
Serve immediately, garnished with grated Parmesan cheese and fresh chopped parsley.