Cook the fettuccine pasta according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of the pasta water.
In a large skillet, heat the olive oil over medium heat. Season the salmon fillets with garlic powder, salt, and pepper. Add the salmon to the skillet and cook for about 4-5 minutes per side, or until cooked through. Remove from the skillet and flake into large pieces.
In the same skillet, add the chopped spinach and cook for 2-3 minutes until wilted.
Pour in the heavy cream and bring to a simmer. Stir in the Parmesan cheese, lemon juice, and red pepper flakes (if using). Mix until the cheese is melted and the sauce is creamy.
Add the cooked fettuccine and flaked salmon to the skillet. Toss to combine, adding reserved pasta water as needed to reach desired sauce consistency.
Serve immediately, garnished with chopped parsley.