In a pressure cooker over medium-high heat, heat the oil. Once hot, add the onion, sausage, and garlic, breaking up the sausage with a spoon and stirring occasionally.
Once the sausage is cooked through, approximately 6 – 8 minutes (feel free to drain the sausage if desired), add the plum tomatoes (crushing them by hand as you add them), black pepper, tomato paste, oregano, crushed red pepper (if using) and beef stock. Stir to combine well.
Pour the pasta over the top of the meat mixture. Using a spoon, gently press the pasta down into the liquid, but not down to the bottom of the pot. Do not stir the pasta into the meat mixture.
Secure the lid into place and bring to high pressure. Cook on high pressure for 9 minutes. Adjust times as needed for your pressure cooker.
After the cook time, turn the heat off and release the pressure using the release valve.
Once pressure is completely released, open the lid away from you and set aside safely. If the pasta needs more cooking time, place the lid on the pressure cooker and cook for another 3 to 5 minutes, or cook the pasta further with the lid slightly ajar on the pressure cooker until the pasta is just al dente.
Add the heavy cream and spinach and stir well, but gently, to combine all the ingredients. Taste for seasoning and add salt and/or pepper as desired.
Serve into individual bowls or pour into a large serving bowl and sprinkle shredded Parmesan over the top.