Cook the fettuccine pasta according to package instructions until al dente. Drain and set aside, reserving ½ cup of pasta water.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until translucent. Stir in the minced garlic and cook for an additional 1 minute.
Add the fresh spinach to the skillet and cook until wilted, about 2-3 minutes.
Pour in the heavy cream and bring to a gentle simmer. Stir in the flaked salmon and lemon juice. Season with salt and pepper to taste.
Add the cooked fettuccine to the skillet, tossing to combine. If the sauce is too thick, add a bit of the reserved pasta water until you reach your desired consistency.
Stir in the grated Parmesan cheese until melted and well combined. Remove from heat.
Serve immediately, garnished with chopped parsley.