In a large pot, bring salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Drain and set aside.

In a skillet, heat olive oil over medium heat. Season the salmon fillet with salt and pepper, then add to the skillet. Cook for about 4-5 minutes on each side, or until cooked through. Remove from skillet and flake into pieces.

In the same skillet, add diced onion and minced garlic. Sauté for 2-3 minutes until the onion is translucent. Add the chopped spinach and cook until wilted.

Pour in the heavy cream and bring to a simmer. Stir in the parmesan cheese and lemon juice. Mix until the cheese is melted and the sauce is creamy.

Add the cooked pasta and flaked salmon to the skillet. Toss everything together until well combined. Adjust seasoning with salt and pepper as needed. Serve hot.