Place a large pot or Dutch oven over medium-high heat. Add the ground Italian sausage and cook until it is browned and crumbled, about 5 to 7 minutes. Use a wooden spoon to break the meat into small pieces as it cooks. Once the meat is nearly done, add the diced onion and cook for another 3 minutes until the onion is translucent. Stir in the minced garlic, oregano, and red pepper flakes. Cook for 1 minute until the garlic becomes fragrant, being careful not to burn it.

Pour in the chicken broth and the undrained diced tomatoes. Stir the mixture well, scraping the bottom of the pot to release any browned bits of sausage, which add immense flavor. Bring the liquid to a gentle boil. Once boiling, reduce the heat to medium-low and let the soup simmer for about 10 minutes. This allows the flavors of the sausage and spices to infuse into the broth.

Turn the heat back up to medium and add the refrigerated cheese tortellini to the pot. Cook the pasta according to the package instructions, usually about 3 to 5 minutes, or until the tortellini are tender and float to the top. Once the pasta is cooked, stir in the fresh baby spinach and heavy cream. The heat from the soup will wilt the spinach in about 1 minute. Stir in the Parmesan cheese until melted. Taste the soup and add salt and pepper as needed. Ladle the hot soup into bowls and serve immediately.