Heat olive oil in a large stockpot or Dutch oven over medium heat. Add Italian sausage and onion, and cook until sausage has browned, about 3-5 minutes, making sure to crumble the sausage as it cooks; drain excess fat.
Stir in garlic and Italian seasoning until fragrant, about 1 minute.
Gradually whisk in beef stock and tomato sauce. Stir in gnocchi. Bring to a boil; reduce heat and simmer until reduced and slightly thickened, and the gnocchi is cooked through, about 6-8 minutes.
Stir in spinach and heavy cream until the spinach has wilted, about 1-2 minutes.
Stir in Parmesan; season with salt and pepper, to taste.
Serve immediately, garnished with basil, if desired.