Add onions, celery and garlic to the same pot and sauté for 7-10 minutes until nicely browned. Pour the wine and cook for 1 minute. Add the tomatoes, tomato purée, thyme, rosemary, bay leaves, stock and chilli flakes. Season with salt and lots of black pepper.
Return the lamb to the pot and stir well. Cover with a lid and cook on a low-flame for 1-11/2 hours, stirring occasionally, and checking a few times in between to make sure it’s not catching at the bottom. (Alternatively, pressure cook the lamb for 15-25 minutes until cooked).