1. Bring a large pot of water to a boil. Add pasta, boil according to package directions and strain.
2. Heat a large skillet over medium heat, and add olive oil.
3. Add the shrimp to the skillet and cook them 1 to 2 minutes. Flip the shrimp, and cook until fully pink and cooked through, about 1 minute more.
4. Add onions, mushrooms and garlic. Cook for 2 minutes and scrape the bottom of the skillet with a wooden spoon.
5. Add the heavy cream and butter, and bring to a simmer and stir gently.
6. Add spinach and cook until slightly wilted. Stir in salt and chili flakes. Lower the heat to keep the sauce warm.
7. Add cooked fettuccine to sauce.
8. Top with Parmesan cheese, if desired.