01 - Slice leeks lengthwise and rinse thoroughly under cold water to remove grit between layers. Slice into thin half-moons and set aside.
02 - Melt butter in a large pot over medium heat. Add leeks and onion; sauté until soft and translucent, approximately 8 minutes.
03 - Stir minced garlic into the pot and cook for 1 minute until fragrant, being careful not to brown.
04 - Add diced potatoes, bay leaf, thyme, and a generous pinch of salt and pepper. Stir to coat evenly with the butter and aromatics.
05 - Pour in vegetable broth and bring to a boil. Reduce heat to maintain a gentle simmer and cook uncovered for 20-25 minutes until potatoes are very tender when pierced with a fork.
06 - Discard the bay leaf from the soup before puréeing.
07 - Using an immersion blender, purée the soup directly in the pot until completely smooth and creamy. Alternatively, transfer to a countertop blender in batches, then return to pot.
08 - Stir in whole milk and heavy cream. Warm through over low heat for 3-5 minutes, being careful not to boil. Adjust seasoning with additional salt and pepper as needed.
09 - Ladle hot soup into bowls and garnish with fresh chives or parsley and croutons, if desired.