Brown the Italian sausage in a large pot or Dutch oven over medium heat. 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 Italian seasoning, oregano, and red pepper flakes, if using. Bring the mixture to a simmer.
Reduce the heat to low, cover, and let it simmer for 10 minutes to allow the flavors to combine.
Stir in the heavy cream and increase the heat slightly to bring the soup back to a gentle simmer. Do not boil after adding the cream.
Add the refrigerated tortellini to the soup. Cook according to package directions, usually about 3 to 5 minutes, until the tortellini float and are cooked through.
Stir in the fresh spinach or kale until wilted. This usually takes 1 to 2 minutes.
Remove the pot from the heat. Stir in the Parmesan cheese until melted and the soup is creamy.
Taste the soup and adjust seasoning with salt and pepper as needed before serving your hearty sausage soup.
Notes
For a slow cooker tortellini soup variation, brown the sausage and sauté the onion and garlic first, then transfer everything except the cream, tortellini, and spinach to the slow cooker. Cook on low for 4-6 hours. Add the cream, tortellini, and spinach during the last 30 minutes of cooking.
You can substitute mild Italian sausage with hot Italian sausage for extra spice.
This recipe makes a fantastic quick dinner soup, often ready in under 45 minutes total.