Prepare the Mashed Potatoes

• Place the potatoes in a large pot and cover with cold salted water.
• Bring to a boil and cook for 15–20 minutes, or until fork-tender.
• Drain well, then mash with butter, warm milk, sour cream, salt, and pepper until smooth and creamy.
• Stir in the cheddar cheese if using. Set aside.

Cook the Meat Filling

• Preheat the oven to 400°F (200°C).
• Heat olive oil in a large skillet over medium heat.
• Add the onion and carrots and cook for 5–6 minutes until softened.
• Stir in the garlic and cook for 30 seconds.
• Add the ground lamb and cook until browned, breaking it apart with a spoon. Drain excess fat if necessary.
• Stir in the tomato paste and flour. Cook for 1 minute.
• Pour in the beef broth and Worcestershire sauce. Add thyme, rosemary, salt, and pepper.
• Simmer for 5–7 minutes, stirring occasionally, until the sauce thickens.
• Stir in the frozen peas and remove the skillet from the heat.

Assemble the Shepherd’s Pie

• Transfer the meat mixture to a baking dish and spread it evenly.
• Spoon the mashed potatoes over the filling, spreading them gently to cover the entire surface.
• Use a fork to create light ridges on top for a golden, crispy finish.

Bake

• Bake for 20–25 minutes, until the filling is bubbling and the potato topping is lightly golden.
• For a deeper golden top, broil for 2–3 minutes, watching carefully.
• Let the pie rest for 10 minutes before serving.