Heat a large pot over medium heat. Add the Italian sausage, breaking it up with a spoon. Cook until browned and cooked through, about 6–8 minutes. Transfer the browned sausage to a bowl and set aside.

In the same pot, add the chopped onion. Cook, stirring occasionally, until the onion is translucent, about 4–5 minutes. Add minced garlic and cook for another 30–60 seconds until fragrant.

Pour in the chicken broth, scraping the bottom of the pot to lift any browned bits. Bring the liquid to a gentle simmer.

Reduce the heat to low and stir in the heavy cream. Add the grated Parmesan gradually, stirring until it melts into a smooth, creamy broth. Return the browned sausage to the pot.

Add the chopped spinach and stir until wilted, about 1–2 minutes. Taste and season with salt and pepper.

Ladle the soup into bowls and serve hot with crusty bread if desired.