Get a large pot of water boiling and cook your pasta as per the instructions on the package. Once done, drain and set aside.
While your pasta cooks, grab a large skillet and heat the olive oil. Toss in the sausage and red pepper flakes, then cook over medium-high heat until the sausage is nicely browned and cooked through.
Add in your onions and garlic, cooking until the onions are tender.
Next, toss in the tomatoes and heavy cream. Let this simmer until your mixture starts to thicken. This should take about 10 minutes.
Add the spinach and let it cook for an additional 3 minutes.
Finally, add the cooked pasta to your sauce and toss until well combined. Sprinkle with parsley, if you like, and serve.