Cut off the dark green part of the top and bottom roots (the white and light green parts should be left).
Cut the leeks in half lengthwise.
Slice them lengthwise into about 1/2" strips.
Wash them really well after slicing (leeks tend to be filled with dirt between the layers and near the root).
To Prepare Soup:
In a large pot, melt the butter (or add cooking oil) over medium heat. Add leeks, garlic, shallots and cook until tender.
Add leeks, garlic, shallots and cook just until tender.
Add all the potatoes, 1 cup vegetable broth, and a few pinches of salt. Combine.
Cover and cook until the potatoes soften. Make sure you add enough broth so that the potatoes don't burn.
Add the remainder of the vegetable broth, bay leaf, dried thyme, lemon juice, salt, and pepper. Cover and bring to a boil. Reduce heat and simmer until the vegetables are very tender (about 30 min.).
Use a hand blender to puree everything in the pot. Thin the soup with additional stock if it is too thick.
Season with salt, freshly ground black pepper, and additional lemon juice (if needed) to taste.
Garnish with green onions and serve hot with your favorite grilled cheese or some homemade garlic bread.
Notes
Vegetable Broth - I always use Imagine Brand low-sodium. Depending on the actual size of your potatoes, you may need some addition vegetable broth if the soup is too thick after pureeing with hand blender. Leftovers heat well the following day.