Preheat the oven to 400°F.

Make the potatoes. In a large stock pot, cover potatoes with 1 inch of cold water. Bring to a boil and cook until the potatoes are tender—a fork should slide in and out effortlessly after about 10 to 15 minutes. Drain potatoes and return to the warm stock pot. Add butter, salt, and garlic. Mix using a hand mixer until well incorporated. Add heavy cream, a little at a time, until you reach your desired consistency. Cover with plastic wrap.

Make the filling. Heat a skillet over medium heat, 3 to 5 minutes. Add olive oil and lamb and season with salt and pepper before browning for 5 to 7 minutes. Remove lamb to a paper towel-lined plate and reserve 2 tablespoons fat in the skillet.

Lower heat to medium-low and add onion, carrots, and garlic. Cook until softened, 8 to 10 minutes. Add tomato paste, thyme, rosemary, Worcestershire sauce, salt, and pepper. Stir until incorporated, then add lamb back to the pan.

Add Guinness and broth and bring to a simmer for 10 minutes, or until the liquid has reduced and the sauce thickens slightly. Add peas and cook for 2 additional minutes. Remove skillet from heat.

Build the Shepherd’s Pie. Add mashed potatoes to a piping bag with a large piping tip. Pipe the mashed potatoes over the filling until the entire dish is covered. Place the skillet in the oven and bake for 20 to 25 minutes or until the top is golden brown. Remove from the oven and serve!