Spray a large frying pan with low calorie cooking spray and place over a medium heat.
Add the onion, garlic, ginger and green chilli and fry gently for 5 - 10 minutes, stirring, until softened and golden.
Add the beef, chilli powder, turmeric, cumin, ground coriander, garam masala and cinnamon to the frying pan. Stir together and fry gently for 2 - 3 minutes to seal the beef on all sides.
Transfer to the slow cooker and stir in the tomatoes, tomato puree, beef stock, vinegar, salt and sweetener until well mixed. Cover with the lid and turn on to the high setting and cook for 4 - 5 hours. The curry will be ready when the beef is tender, and the sauce has reduced and thickened.
Serve sprinkled with chopped coriander leaves if wished and serve with some basmati rice or other accompaniments of your choice.