01 - Melt butter in a large soup pot over medium heat. Add onion, leek, carrots, and celery. Sauté for 5 minutes until vegetables soften.
02 - Stir in diced potatoes and cook for an additional 2 minutes, allowing ingredients to blend.
03 - Pour in vegetable broth and add bay leaf, white pepper, nutmeg, and salt. Mix well.
04 - Bring mixture to a boil, then reduce heat to low. Cover and simmer for 25 minutes, or until all vegetables are tender.
05 - Remove bay leaf. Partially mash the soup using a potato masher or immersion blender for a creamy consistency, leaving some potato pieces intact.
06 - If using sausage, stir in slices and simmer for 5 additional minutes.
07 - Taste and adjust salt as needed. Serve hot, garnished with fresh parsley.