Using the Instant Pot’s Sauté setting, heat the olive oil. Brown the Italian sausage and cook the onion together in the pot, breaking the meat up into small crumbles. When the sausage is cooked, add the garlic and sauté for an additional minute.
Add a splash of chicken broth and scrape the bottom of the Instant Pot to deglaze if needed, scraping the bottom of the pot with a flat spatula. This helps to prevent a burn error. Then, add the remaining broth, the tortellini, diced tomatoes (do not drain), Italian seasoning, black pepper, and red pepper flakes. Do not stir, but gently press the tortellini to submerge it.
Cancel the Sauté, close the lid, and turn the knob to Sealing. Set the Instant Pot to Pressure Cook on High for 2 minutes. It will take the pot about 10-15 minutes to come to pressure before the cooking time begins.
When the cooking time is complete, carefully quick release the pressure. If the steam release valve spits liquid, close the valve and wait 3-5 minutes before finishing the quick release.
After the pressure is released, remove the lid and stir in the baby spinach. Allow the spinach to wilt (about 1-2 minutes) before stirring in the heavy cream. Serve with shredded parmesan (optional).