Place the diced carrots, onions, garlic and celery in an ovenproof saucepan with the olive oil and cook until lightly coloured, about 3-4 minutes.
Add the bacon, thyme, bay leaf and lentils then cover with hot chicken stock and bring to the boil. Cover with a piece of buttered greaseproof paper and then put a piece of foil on top. Place in the oven and cook for 15-20 minutes.
Spoon some of the lentils out and place to one side as a garnish. Remove the bay leaf, thyme and bacon and discard. Place the soup in a food processor and puree to smooth consistency.
Heat the cream and add to the soup, then pour the soup through a sieve for even smoother consistency. Season to taste.
Divide the soup into bowls and sprinkle the remaining lentils on top. Garnish with watercress and a spoon of creme fraiche.