Heat a large pot or Dutch oven over medium-high heat. Add the olive oil and crumble the Italian sausage into the pot. Cook until browned and cooked through, breaking it up with a wooden spoon as it cooks (about 5–7 minutes).
Use a slotted spoon to remove the sausage from the pot, leaving behind any rendered fat. Set the sausage aside on a plate.

In the same pot, add the diced onion to the remaining sausage fat. Sauté for 3–4 minutes, or until softened and translucent.
Stir in the minced garlic and cook for an additional 30 seconds, until fragrant.

Add the diced potatoes and sliced carrots to the pot. Stir to combine with the onions and garlic.
Pour in the chicken broth and stir well. Add the Italian seasoning , red pepper flakes (if using), and season with salt and black pepper to taste.
Return the cooked sausage to the pot and stir everything together. Bring the mixture to a boil, then reduce the heat to medium-low and simmer for 15–20 minutes, or until the potatoes and carrots are tender.

In a small saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1–2 minutes, stirring constantly, to create a roux.
Gradually whisk in the milk , ensuring there are no lumps. Cook, stirring frequently, until the mixture thickens slightly (about 3–5 minutes). Remove from heat.

Stir the creamy mixture into the pot of soup, mixing well to combine. Simmer for an additional 5 minutes to allow the flavors to meld.
Add the fresh spinach to the pot and stir until wilted (about 2–3 minutes).

Taste the soup and adjust the seasoning with additional salt , black pepper , or red pepper flakes if desired.
Ladle the soup into bowls and serve hot. Garnish with grated Parmesan cheese, fresh parsley, or crusty bread on the side for an extra touch