Heat the olive oil in a large pot or Dutch oven over medium heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned. Drain off excess grease.
Add the chopped onion to the pot and cook until softened, about 5 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the chicken broth and diced tomatoes (with their juice). Add the oregano, basil, and red pepper flakes, if using. Bring the mixture to a simmer.
Reduce the heat to low, cover, and let the soup simmer for 10 minutes to allow the flavors to combine.
Stir in the heavy cream and increase the heat to medium-low.
Add the refrigerated tortellini to the soup. Cook according to package directions, usually about 5 to 7 minutes, until the tortellini float and are cooked through.
Stir in the fresh spinach until it wilts into the soup.
Remove the pot from the heat. Stir in the grated Parmesan cheese until melted and the soup is creamy.
Taste the soup and add salt and black pepper as needed before serving.