Cook the Potatoes:
Place peeled potato chunks in a large pot of salted water. Bring to a boil and cook until tender, about 15–20 minutes. Drain well, then mash with butter, warm milk, cheese (if using), and season with salt and pepper. Set aside.
Prepare the Meat Filling:
In a large skillet or pan, heat olive oil or butter over medium heat. Add onions and carrots, cooking until softened, about 5 minutes. Stir in garlic and cook for another minute.
Brown the Meat:
Add ground lamb (or beef) to the pan, breaking it up with a spoon. Cook until browned and no longer pink. Drain excess fat if needed.
Build Flavor:
Stir in tomato paste, Worcestershire sauce, thyme, and rosemary. Sprinkle the flour over the mixture and stir well. Slowly pour in the beef broth, stirring constantly until thickened into a rich gravy. Add peas last, stirring them into the mixture. Season with salt and pepper to taste.
Assemble the Shepherd’s Pie:
Preheat oven to 400°F (200°C). Spread the meat mixture evenly into the bottom of a 9x13 baking dish. Spoon the mashed potatoes over the top and spread them evenly, sealing the edges. Use a fork to create ridges on the potatoes (this helps them brown).
Bake to Perfection:
Place the dish in the oven and bake for 20–25 minutes, or until the potatoes are golden brown and slightly crisp on top.
Cool and Serve:
Let the shepherd’s pie rest for 10 minutes before serving. Enjoy this hearty, comforting dish with a side salad or steamed vegetables.