For the Potatoes:

1. To a large pan add the potatoes, add cold water until the potatoes are covered and bring to a boil.
2. Turn down to a low boil and cook for 20 minutes until tender.
3. Drain the potatoes a colander, and add back to the pan.
4. Mash until smooth and add the butter, milk, salt and pepper to taste.
5. Mix well and set aside.

For the Filling:

1. Preheat oven to 350°F.
2. Add vegetable oil to a 10-inch cast iron skillet over medium-high heat and add 1/2 of the ground lamb. If the pan is too crowded, the meat will not brown, it will boil. Season with half of the salt and pepper, sauté until browned, breaking up the meat as it browns.
3. Drain the meat keeping the fat in the pan and transfer to bowl or plate and repeat with the other 1/2 of the lamb, salt and pepper.
4. Turn the heat down to medium and sauté the onions for about 5 minutes until softened.
5. Add the garlic and thyme, cook for 3 minutes. Add tomato paste and mix well.
6. Add the carrots, parsnip, peas, Guinness, Worcestershire sauce, beef stock and cooked beef.
7. Simmer for 20-25 minutes until the liquid has reduced and the carrot and parsnips are tender. Taste for seasoning and add salt and pepper if needed. Turn off the heat.
8. Spread the potato mixture (or pipe with a piping bag for a decorative look) over the ground meat mixture. Make sure you make little peaks so they get a nice brown color in the oven.
9. Place the pan onto a baking sheet and bake in the oven for 25 minutes or until the top is golden brown.
10. Remove from the oven and allow to sit for 10 minutes before serving.