• Preheat oven to 400°F. Lightly butter or spray a casserole dish on the bottom and sides. Line a large, rimmed baking sheet with parchment paper or aluminum foil. Baking the casserole atop a lined tray will help to catch any drips which may bubble-over. Set aside. Prepare a large pot of water by adding Kosher salt. Also set aside.

prepare the lamb filling

• In a large skillet over medium heat, add oil and heat to shimmering. Add garlic and immediately adjust the heat to avoid burning the garlic. Cook for 6 minutes. Chop two medium onions and a large carrot. Alternatively, you may use par-boiled, trimmed whole pearl onions if on hand. Add them to the skillet and cook for 8 minutes.
• Adjust the stove top to medium-high heat. Add tomato paste, stirring constantly. Reduce heat slightly and cook until the color reaches a deep reddish brown, about 10 minutes. Raise the heat to medium-high and add the lamb. Cook until the ground meat is nicely browned, about 10 minutes.
• Add the Guinness and scrape the bottom of the pan with a wooden spoon of the browned bits. Next, add the brown sugar, chicken stock, Worcestershire, thyme and rosemary, and season liberally with Kosher salt and pepper. Simmer over medium-low heat until the juices in the meat mixture are 90% reduced. The liquid that remains should appear syrupy. Remove from the heat and allow to cool slightly while you turn attention to the potato mash.

for the whipped potatoes

• Cook until the potatoes in the boiling water until they are fork tender when tested, anywhere from 10 to 15 minutes. Once tender, drain the potato water to stop the cooking process. Spread the potatoes in an even layer onto a baking sheet. Let sit for 5 minutes allowing them to steam out. Once you stop seeing steam, run the potatoes through a ricer.
• While the potatoes are boiling, in a medium saucepan over medium heat, sauté some garlic in butter for 5 minutes, again adjusting the temperature to prevent the garlic from burning. Add in cream and turn heat to low until the potatoes are finished cooking. Using a wooden spoon, gently fold the riced potatoes into the cream mixture. You want to NOT overwork the potatoes. Season to taste with Kosher salt and black pepper.

assembly

• Stir the peas into the lamb mixture. In the prepared baking dish, spread the ground lamb evenly on the bottom of the dish. Alternatively, this may be baked in a large oven-safe skillet or cast iron skillet. Top with potatoes and spread evenly as well. Use the tines of a fork to drag the top of the potatoes making a pattern if desired.
• Place the casserole atop the prepared baking sheet. Bake for 20-25 minutes until the potatoes have browned on top and the gravy is bubbling from beneath. Remove from the oven, garnish with parsley and serve immediately. The pie will be VERY HOT.