01 - Melt unsalted butter in a large pot over medium heat. Add sliced leeks and chopped onion, cooking gently for 5 to 7 minutes until softened without browning.
02 - Incorporate minced garlic and cook for 1 minute until fragrant.
03 - Add diced potatoes, bay leaf, and dried thyme to the pot. Season with salt and freshly ground black pepper.
04 - Pour in vegetable broth, bring to a boil, then reduce heat to simmer uncovered for 20 to 25 minutes until potatoes are very tender.
05 - Take out and discard the bay leaf from the soup.
06 - Using an immersion blender, blend the soup until smooth and creamy. Alternatively, carefully purée in batches using a regular blender.
07 - Stir in whole milk or heavy cream, heating gently until warmed through. Adjust seasoning with salt and pepper as needed.
08 - Ladle the soup into bowls and garnish with chopped chives, parsley, croutons, or a drizzle of extra cream, according to preference.