Put on a pot of potatoes to boil. When potatoes are done, mash with the stick of butter and set aside. The potatoes should be light and fluffy.

While the potatoes are cooking heat a 10" cast iron skillet to medium-high heat. Brown the lamb well then remove and set aside. Add the onion, celery, and carrots, diced, and cook until softened. Add herbs and mix well, then add the stock, tomato paste, worcestershire, salt, (about a tsp) and pepper (a few grinds). Cook until the mixture reduces down a bit and thickens some. Add the peas and top carefully with the potato mixture, globbing it on then spreading gently and evenly. Score or fluff the top to create peaks and valleys, ensuring there is a tight seal all around the pan. It can help to scrape potato towards the edge to help the seal.

Broil until the potatoes on top are golden brown, about 5-10 minutes. Use a sheet pan under the cast iron pan as the filling may bubble over.