Bring a large pot of salted water to a boil. Add fettuccine and cook according to package instructions until al dente. Drain and set aside.

In a skillet, heat olive oil over medium heat. Add crumbled Italian sausage and cook until browned, about 5-7 minutes.

Add chopped onion and minced garlic to the skillet with the sausage. Sauté until the onion is translucent, about 3-4 minutes.

Pour in the heavy cream and stir in the Italian seasoning, red pepper flakes, salt, and black pepper. Bring to a simmer and let it thicken slightly, about 5 minutes.

Add the cooked fettuccine and fresh spinach to the skillet. Toss to coat the pasta in the sauce and heat through, about 2-3 minutes.

Serve hot, topped with grated Parmesan cheese.