Heat olive oil in a large Dutch oven over medium-high heat. Add the Italian sausage and cook 5–7 minutes, stirring to break up pieces, until golden and crackling.

Add the onion, carrots, and celery. Sauté 5 minutes until vegetables are tender with slight bite and the edges turn translucent.

Stir in the garlic and cook 30 seconds until fragrant steam rises, then stir in the butter until melted.

Sprinkle the flour over the mixture and whisk constantly for 1–2 minutes to form a pale roux.

Gradually pour in the chicken broth, whisking to prevent lumps. Stir in the milk, bring to a gentle simmer, then reduce heat to medium-low.

Fold in the cannellini beans and spinach. Simmer 3–5 minutes until heated through and spinach is just wilted.

Stir in the heavy cream and Parmesan cheese. Cook 1–2 minutes, stirring until the cheese melts and the soup thickens slightly. Season with salt, pepper, and red pepper flakes.