Sauté the bacon in a deep saucepan for 5 minutes. Add the olive oil, leek and onion and sauté for 3 minutes. Stir in the flour, then slowly add the stock, stirring constantly until thickened. Add the potatoes and simmer for 35 minutes. Remove from the heat, pour the mixture into a blender and blend until smooth and creamy. Return the soup to the saucepan and bring to the boil. Add the mustard and adjust the seasoning to taste. Combine the egg yolks and sour cream and place a little in each of 6 small serving dishes. Pour the remainder into the soup and lightly stir in. Ladle the soup into 6 bowls, arrange on plates with the dishes of sour cream mixture alongside and sprinkle the soup with chopped chives. Serve the soup with fresh ciabatta, or brush the slices of ciabatta with olive oil and chargrill on a grill or barbecue.