Dice carrot, celery, apple, and onion. Set apple aside. Heat oil over medium heat. Add carrot, celery, and onion and cook 2 minutes. Add butter and stir to melt. Add stock and cook for 10 minutes. Add apple, tomato paste, bay leaf and sugar. Cook for 20 minutes longer. Remove bay leaf. Transfer soup to blender or using immersion blender, blend until smooth. Strain soup over fine sieve, keeping liquid and discarding any solids. Season with salt and pepper and serve hot.