Preheat the oven to 200C Fan.
Peel and chop the potatoes. Place them into a pan of cold water, season with salt then bring to the boil. Simmer until they are tender.
Meanwhile, in a large pan, heat some cooking oil over a medium to high heat. Cook the onion, carrot, and celery for about 5 minutes, or until it softens.
Add in the minced lamb and cook until it’s browned all over.
Sprinkle in the flour, stir well and cook for a further minute.
Pour in the Guinness, and Worcestershire sauce. Stir well and simmer for about 5 minutes. Taste for seasoning.
Chuck in the frozen peas, give it a stir, then tip the mince into an oven proof baking dish.
Once the potatoes are tender, drain them and leave them to steam dry.
Mash them, then add in the butter and cheese, and mix until you have a smooth mash. Pour in the milk and stir well. Taste for seasoning.
Spoon the mash over the mince, smooth it over, then use a fork to rough it up a bit. Sprinkle over the cheese, then bake for 20 minutes.
Take it out from the oven and leave to stand for a few minutes before serving.
Enjoy!!