Spray the inner pot of the Instant Pot with low-calorie cooking spray and turn on to 'sauté'. Add the onions and sauté for about 5 minutes until softened. Add the garlic and sauté for a further 1-2 minutes.
Add the lamb to the Instant Pot and sauté for 2-3 minutes to seal and lightly brown on all sides.
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.
Place the lid on the Instant Pot and lock in place. Press 'pressure cook' and set the timer for 30 minutes on high pressure. Cook for 30 minutes and when finished, use natural pressure release. When the float valve has gone down, open the lid.
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.