In a large heavy pot heat 3 tablespoons oil and saute the leeks until tender, about 5 to 8 minutes. Reserve 3 large cloves of the garlic. Add the remaining garlic and sweet potatoes to the pot and stir to combine. Add enough chicken stock or water to barely cover the vegetables. Season with salt and pepper. Bring to a boil, reduce heat and simmer until the sweet potatoes are very tender about 20 to 30 minutes.
Cut the reserved 3 cloves of garlic into very thin slices. In a small skillet heat the remaining 2 tablespoons oil and cook the sliced garlic until golden. Transfer garlic chips to paper towels to drain.
In a blender puree the soup, in batches, until smooth and transfer to another pot. Taste soup and adjust seasoning if necessary. Return soup to a simmer. Ladle soup into serving bowls and garnish with garlic chips, crumbled feta cheese and chives.
Recipe courtesy of Gourmet Magazine