In a large pot or Dutch oven, heat the olive oil over medium heat. Once hot, add the diced onion and sauté for about 3-4 minutes, or until translucent.
Add minced garlic to the pot and sauté for an additional minute until fragrant.
Remove the sausage from its casing and crumble it into the pot. Cook for 5-7 minutes, stirring occasionally, until browned and cooked through.
Pour in the chicken or vegetable broth and add the can of diced tomatoes along with their juices. Stir in the Italian seasoning and season with salt and pepper to taste.
Bring the soup to a gentle boil, then reduce the heat to low. Allow it to simmer for about 10 minutes to meld the flavors together.
Add the cheese tortellini to the pot and cook according to the package instructions, usually about 3-5 minutes.
In the last couple of minutes of cooking, add the fresh spinach, stirring until wilted and incorporated into the soup.
Taste and adjust seasoning if necessary, serving hot with a sprinkle of grated Parmesan cheese on top.
Notes
Adjust the seasoning to taste and serve hot with grated Parmesan cheese.