Step 1: Brown the Sausage

In a large pot or Dutch oven over medium heat, add the Italian sausage. Cook, breaking it apart with a wooden spoon, until browned and cooked through, about 5-7 minutes. Remove the sausage from the pot and set it aside, leaving the rendered fat in the pot.

Step 2: Sauté the Vegetables

In the same pot, add the chopped onion, minced garlic, sliced carrots, and sliced celery. Sauté for about 5 minutes until the vegetables are softened and the onion is translucent.

Step 3: Add Potatoes and Broth

Stir in the diced potatoes, chicken broth, diced tomatoes (with their juices), dried oregano, and dried basil. Bring the mixture to a boil, then reduce the heat to low and let it simmer for about 20 minutes, or until the potatoes are tender.

Step 4: Return Sausage and Add Spinach

Once the potatoes are tender, return the cooked sausage to the pot. If using, stir in the fresh spinach and cook for an additional 3-5 minutes until the spinach is wilted. Season with salt and pepper to taste.

Step 5: Serve

Ladle the soup into bowls and sprinkle with grated Parmesan cheese before serving. Enjoy this hearty soup with crusty bread or a side salad for a complete meal.