Melt 1 tbsp butter in a nonstick saute pan over med. heat. Add the sliced onion, season with salt and pepper, and saute until the onions are golden brown, 25 to 30 mins. Add 2.1/2 cups chicken stock and the wine. Bring to a boil. Lower the heat and simmer very slowly for 15 mins.
Heat the vegetable oil and water in a 5-qt. saucepan over medium heat. When hot, add the chopped onion, celery, and leeks. Season with salt and pepper and saute for 5 mins. Add the remaining 3 cups chicken stock.
Tie thyme and bay leaves in a small piece of cheesecloth and add to the stock and vegetables. Bring to a boil, lower the heat, and simmer for 25 to 30 mins. Melt the remaining 4 tbsp butter in a 2.1/2-qt. saucepan over low heat. Add the curry powder and whisk to blend. Add the flour, to make a roux, and cook until the roux bubbles, 6 to 8 mins. Stir constantly to prevent browning and scorching. Strain 2 cups simmering stock from the 5-qt. saucepan into the roux. Whisk vigorously until smooth, then add to the remaining stock and chopped vegetables. Whisk until well combined. Bring to a boil, then reduce the heat and allow to simmer for 10 mins.
Remove from the heat. Remove and discard the tied herbs. Puree the soup in a food processor fitted with a metal blade. Strain. Return to low heat in a 5-qt. saucepan. Add the sliced onion and stock mixture. Hold at a simmer.
Acidulate 4 cups water with 1 tbsp lemon juice. Peel, core, quarter, and slice the apples widthwise. Place in the acidulated water as sliced, to prevent discoloration. Heat the cream in a nonstick saute pan over medium heat. Drain the sliced apples in a colander. Rinse under cold running water and shake dry. When the cream is warm, add the apples and heat until the apples are hot, 3 to 4 mins., do not boil. Add to the soup, adjust the seasoning with salt and pepper, and serve immediately.
This soup may be held hot in a double boiler for up to one hour.
Note - this soup is enhanced if it is refrigerated for 24 hours before use, allowing the flavors to develop fully. If you decide to do this, cool the soup quickly, cover and refrigerate for 24 to 48 hrs.
When ready to serve, heat the soup to a simmer, then add the separately heated cream and apples.