01 - Slice leeks lengthwise and rinse thoroughly under running water to remove grit between layers. Slice into thin half-moons.
02 - Melt butter in a large pot over medium heat. Add prepared leeks and diced onion; sauté until soft and translucent, approximately 8 minutes.
03 - Stir in minced garlic and cook for 1 minute until fragrant, being careful not to brown.
04 - Add diced potatoes, bay leaf, dried thyme, and a generous pinch of salt and black pepper. Stir well to coat vegetables.
05 - Pour in vegetable broth and bring to a boil. Reduce heat to low and simmer uncovered for 20-25 minutes until potatoes are very tender when pierced with a fork.
06 - Discard the bay leaf from the pot.
07 - Using an immersion blender, purée the soup directly in the pot until completely smooth and creamy. Alternatively, blend in batches using a countertop blender and return to pot.
08 - Stir in whole milk and heavy cream. Warm through over low heat for 2-3 minutes, being careful not to boil. Taste and adjust seasoning with additional salt and pepper as needed.
09 - Ladle hot soup into serving bowls. Top with fresh chives or parsley and croutons if desired. Serve immediately while hot.