Spray a large frying pan with low calorie cooking spray and place over a medium heat. Add the onions and fry, stirring, for 10 minutes or until softened and golden brown. Transfer the onions to the slow cooker.
Add the lamb and garlic to the frying pan and cook over a medium heat for 2 - 3 minutes to seal the lamb on all sides. Transfer to the slow cooker.
Add the potatoes, leeks, carrots, pearl barley, stock, thyme and bay leaves to the slow cooker. Season with salt and pepper and stir to combine well.
Cover with the lid and turn the slow cooker on to the low setting. Cook for 4 1/2 - 5 hours, stirring once halfway through if possible. Remember not to keep lifting the lid , each time you do will add another 30 minutes onto the cooking time.
Remove the bay leaves and adjust the seasoning before serving.