In a large pot or Dutch oven, cook the Italian sausage and diced onion over medium heat until the sausage is browned and the onion is softened. Break the sausage apart as it cooks.
Add the minced garlic and cook for 1 minute, stirring often.
Pour in the chicken broth, then stir in the Italian seasoning, salt, pepper, and red pepper flakes. Bring the soup to a gentle boil.
Add the gnocchi and cook according to the package directions, usually 3-5 minutes, or until the gnocchi is tender and floats to the top.
Reduce the heat to low and stir in the heavy cream, spinach, and parmesan cheese.
Simmer for 2-3 minutes, until the spinach is wilted and the soup is creamy.
Taste and adjust the salt, pepper, or red pepper flakes if needed before serving.