Place potatoes in a large saucepan then cover with water and add a pinch of salt.
Place on high heat.
When water begins to boil, lower to medium heat and allow to cook.
Cook for approx. 15 minutes, until you able to pierce each potato easily with a fork.
Drain potatoes and return them to saucepan.
Add butter and half and half, and then mash vigorously with masher until free of lumps.
Season mashed potatoes with salt and pepper, then cover and set aside once done.
Heat 2 tsp of oil in skillet, then add half of the ground lamb.
Cook lamb until water is released and then browned, moving meat around the pan to avoid sticking
Once cooked and browned tip lamb into a colander with a bowl underneath it to catch the oil.
Shake colander to catch all of the oil then set lamb aside and return oil to skillet
Add 1 tsp of cooking oil to skillet, then repeat process with balance of ground lamb, remembering to drain and return oil to skillet.
Place pan on medium heat then add carrots, celery, onions and peas, mix together thoroughly then cover. Use foil if your skillet has no cover.
Lower heat to low and allow to cook for 10 mins.
Whisk together stock, Worcestershire sauce and tomato puree and set aside.
Raise heat to medium then add browned lamb and garlic then mix thoroughly.
Add Guinness and stock mixture then mix throughly again.
Add salt and pepper to taste, then mix throughly once more.
Cover skillet and lower heat to low and cook for approx. 20 minutes, or until vegetables are cooked.
If you're not using a cast iron skillet, transfer ground lamb to oven proof dish before patting down into place.
Top with mashed potatoes then score with a fork horizontally and then vertically.
Cook in preheated oven at 375 for 15-20 mins, or until potato becomes golden.