1. Peel and dice the potatoes, onion, carrot, and celery.

2. In a large saucepan, melt the butter over medium heat. Add the diced vegetables and season with the salt and pepper. Cook, stirring often, for about 8 minutes, or until the vegetables are tender.

3. Pour in the stock and bring to a boil. Reduce the heat and simmer for 15 minutes.

4. In a small bowl, whisk together the milk and flour until smooth.

5. Slowly pour the milk mixture into the soup and stir until thickened.

6. Add the parsley and sour cream. Simmer for 5 minutes.

7. Taste and adjust seasoning if necessary. Serve hot.