Step 1 Remove the roots and tough outer layers of 4 of the leeks, slice them lengthwise and rinse thoroughly. Slice thinly. In a medium heavy-bottom pot, heat 3 tablespoons of the oil over medium-low heat and cook the leeks and onions just until they soften. Add the garlic and cook another minute, stirring.
Step 2 Stir in the chopped apples, potatoes and water. Stir in the bay leaves, parsley and thyme leaves. Bring to a boil and cook until the vegetables are soft, 45 minutes to an hour.
Step 3Remove bay leaves from the pot and puree the thyme and parsley together with the soup using a blender or food processor (this will need to be done in batches). Season to taste with salt, pepper and nutmeg. Return to the pot and keep covered over low heat.
Step 4To make the garnish, heat the oven to 400 degrees. Clean and slice the remaining 3 leeks as you did with the first 4. Line a baking sheet with parchment paper. Toss the 3 sliced leeks in a bowl with the remaining 2 teaspoons of oil. Transfer to the prepared baking sheet and bake, stirring every few minutes, until the leek slices are crisp, about 10 minutes.
Step 5Divide the soup between serving bowls and garnish each with crispy leeks and a drizzle of olive oil, if desired.