Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onions and fry gently for 5 minutes until softened. Add the garlic and fry for a further 1-2 minutes.
Add the lamb to the frying pan and fry for 2-3 minutes to seal and lightly brown on all sides.
Place the sealed lamb, onions and garlic in the slow cooker. Add the chopped tomatoes, lentils, carrots, mushrooms, stock, tomato puree, mustard, Worcestershire sauce, rosemary, thyme and bay leaf. Season well with salt and black pepper, then stir all the ingredients together until well mixed.
Cover with the lid and cook for 4-5 hours on high. The stew will be ready when the lamb is tender and you have a thick, rich gravy.
Remove the bay leaf and serve with Creamy Mashed Potatoes or an accompaniment of your choice.