Make the Mashed Potatoes:
Place the peeled and chopped potatoes in a large pot and cover with water. Bring to a boil and cook for 15-20 minutes, or until tender when pierced with a fork.
Drain the potatoes and return them to the pot. Add the butter and milk, and mash until smooth and creamy. Season with salt and pepper to taste. Set aside.
Prepare the Meat Filling:
In a large skillet, heat a little oil over medium heat. Add the ground lamb (or beef) and cook, breaking it apart with a wooden spoon, until browned, about 5–7 minutes.
Add the onion, carrots, and garlic to the skillet, and cook for an additional 5 minutes until the vegetables soften.
Stir in the tomato paste, Worcestershire sauce, and thyme.
Pour in the beef broth and simmer for 5–7 minutes until the sauce thickens and the vegetables are tender.
Add the peas and stir to combine. Season with salt and pepper to taste.
Assemble the Shepherd’s Pie:
Preheat the oven to 400°F (200°C).
Spread the meat filling evenly in the bottom of a 9x13-inch baking dish.
Spoon the mashed potatoes on top of the meat mixture, spreading them evenly to cover the entire dish.
Use a fork to lightly score the top of the mashed potatoes to create a textured surface, which will brown nicely in the oven.
Bake the Shepherd’s Pie:
Place the baking dish in the oven and bake for 20–25 minutes, or until the top is golden brown and slightly crispy.
Optional: For extra browning, broil the pie for 1–2 minutes at the end of baking, keeping a close eye to prevent burning.
Serve:
Let the Shepherd’s Pie cool for a few minutes before serving. It pairs wonderfully with a simple side salad or steamed vegetables.