Instructions:
- Prepare the Potatoes: Boil the peeled and quartered potatoes in salted water until fork-tender (approx. 15-20 minutes). Drain thoroughly and allow to steam dry for 5 minutes.
- Mash the Topping: Mash potatoes thoroughly, incorporating the butter, warmed milk/cream, egg yolk, and seasoning until smooth and fluffy. Set aside.
- Sauté Aromatics: Heat oil in a large saucepan or Dutch Oven. Sauté the diced onions, carrots, and celery over medium heat until softened (about 8-10 minutes). Add garlic and cook for 1 minute more until fragrant.
- Brown the Lamb: Push vegetables to the side. Add the ground lamb and brown thoroughly, breaking it up with a wooden spoon. Drain off any excessive fat.
- Build the Gravy Base (Roux): Sprinkle the flour over the meat mixture, stirring constantly for 1 minute. Stir in the tomato paste.
- Deglaze and Simmer: Pour in the stock and Worcestershire sauce. Stir well to scrape up any browned bits from the bottom. Add herbs, salt, and pepper.
- Reduce Filling: Bring to a gentle simmer. Cook, uncovered, for 20–25 minutes, stirring occasionally, until the gravy has visibly thickened and reduced substantially. Stir in the frozen peas during the final 5 minutes.
- Assemble the Pie: Preheat oven to 400°F (200°C). Pour the meat filling into a 9x13 inch baking dish. Spoon the mashed potatoes evenly over the top.
- Finish the Topping: Use a fork to create peaks and texture across the potato surface. Sprinkle with optional grated cheese.
- Bake: Bake for 25–30 minutes, or until the filling is bubbling hot around the edges and the potato topping is deeply golden brown.
- Rest and Serve: Let the pie rest for 10 minutes before carving—this helps the filling set up nicely.