Heat olive oil in a large pot over medium-high heat. Crumble in the sausage and cook until browned, 6 to 8 minutes. Transfer to a plate with a slotted spoon.

Lower the heat to medium, add the onion and cook until soft, 4 to 5 minutes. Stir in the garlic and cook 30 seconds.

Pour in the broth, add the tomatoes with their juices, and return the sausage to the pot. Bring to a boil, then simmer uncovered for 10 minutes.

Remove from the heat. Stir in the heavy cream, then gradually add the Parmesan, stirring until completely smooth.

Add the spinach and stir until wilted, about 30 seconds. Season with salt, pepper, and red pepper flakes if using. Serve immediately.