Preheat oven to 425°. In a large, heavy roasting pan, combine the leeks, carrots, zucchini, eggplant, tomatoes, and potatoes. Add 1/2 C. of the stock, the oil and salt and pepper to taste and mix until the vegetables are well coated. Roast until the vegetables are softened, turning occasionally to make sure they do not burn, about 40 minutes total. Cool somewhat.
In a blender, in batches, combine the vegetables with 1/2 C. of the stock and purée until smooth. Transfer to a large saucepan over low heat and stir in the remaining 3 1/2 C. stock, the basil and the lemon juice. If needed, add a bit more stock for the desired consistency. Simmer for about 3 minutes to allow the flavors to blend. Season with salt and pepper.
Ladle the soup into warmed bowls and serve immediately. Soup can also be made ahead of time, refrigerated, and gently reheated. Makes 8 servings.