Chop the onions and peel and chop the carrots. In a large pan, heat the oil, then gently cook the onion, carrots and thyme for a few minutes until softened.

Once the vegetables are soft, add the lamb to the pan and brown all over, stirring as you go. If you'd like to make this recipe as healthy as possible, drain some of the fat off at this point before continuing with the recipe instructions.  Make up 500ml of stock and add to the pan with the Worcestershire sauce.

Bring to a simmer, then cover the pan and cook for 20 minutes. After 20 minutes remove the lid, add the peas to the pan and cook for a further 20 minutes or until the liquid has been absorbed. Season to taste with salt and plenty of black pepper.

Whilst the meat is cooking, preheat the oven to 180°C (gas mark 4/350°F).  Prepare the mash: peel and chop the potato and sweet potato and cook in boiling water for 15 minutes or until soft. Drain the potatoes well, then return to the pan and mash with the butter and milk.

To assemble the shepherds pie, spoon the mince mixture into an ovenproof dish then top with the mash and spread out to cover evenly. Make a ridged pattern with a fork and pop in the oven to bake for 20-25 minutes, until piping hot with bubbling mince.