01 - In a large soup pot over medium heat, melt the butter. Add the onion, leek, carrot, and garlic. Sauté until softened but not browned, approximately 5 minutes.
02 - Add the potatoes, bay leaf, dried thyme, a generous pinch of salt, and black pepper. Stir to coat all vegetables evenly in the butter and aromatics.
03 - Pour in the stock and bring to a boil. Reduce heat, cover, and simmer for 20 to 25 minutes, or until potatoes are very tender.
04 - Remove the bay leaf. Using an immersion blender, blend the soup until smooth, or leave some texture if preferred.
05 - Stir in the milk and gently reheat without boiling. Adjust seasoning with additional salt and pepper as needed.
06 - Ladle into bowls and garnish with chives, parsley, bacon, or cheese as desired. Serve hot with crusty bread.