Apple, Potato and Leek Soup

In a large pot, melt 1 tablespoon butter over high heat. Add diced onion and leek, cooking until translucent, about 15 minutes. Add the potatoes, all but 2 cups of the diced apples and chicken stock. Season with salt and pepper to taste. Bring to a simmer and cook until tender, about 45 minutes. Pour soup into a colander over a pot to separate the vegetables from the stock (cooking liquids). Ladle the vegetables into a blender or food processor. Add enough stock to cover; puree until smooth. Pour into a fine sieve and strain into another pot. Add more stock if the soup is too thick. Add lemon juice; season with salt and pepper to taste. In a medium nonstick pan, heat the remaining butter over high heat. Add remaining apples, cooking until slightly brown. Sprinkle 1 tablespoon sugar over apples and continue cooking until golden. Add vinegar and remove from heat. Ladle the hot soup into a tureen or serving bowl. Spoon the diced apple garnish into the center of the soup. Garnish the top of the soup with chives.