This is a ultra-smooth pureed vegetable soup based on the French soup called potage. For the silkiest texture, puree it in a blender. You can also use an immersion blender, or even leave it chunky if you prefer. Serve it with the croutons or warm crusty bread.
Heat the butter in a stock pot over medium heat. Add the onion, leeks, carrot and garlic and cook until the onions turn translucent but do not brown, about 10 minutes. Add the potatoes and stir in the thyme and salt.
Pour in the stock and add just enough water to cover the vegetables. Add the bay leaf and bring to a simmer and cook, partially covered until the potatoes are fork tender, about 20 minutes.
Puree the soup in the pot with an immersion blender. For the smoothest texture, puree in a blender and transfer back into the pot.
Stir in the milk, if using, taste for seasoning and serve with the croutons, if desired.
For a quick vegetable stock, simmer 4 cups of water with the green parts of the leek along with the trimmings from the onions, garlic and carrots. Simmer for 15-20 minutes and strain to use for this soup.