Step 1: Prepare the Mashed Potatoes

1. In a large pot, add the peeled and cubed potatoes. Cover with cold water and add a pinch of salt. Bring to a boil and cook until tender (about 15-20 minutes).
2. Drain the potatoes and return them to the pot. Add milk, butter, salt, and pepper. Mash until smooth and creamy. Set aside.

Step 2: Cook the Filling

1. In a large skillet, heat olive oil over medium heat. Add chopped onion, carrots, and celery. Sauté until softened (about 5-7 minutes). Add minced garlic and cook for an additional minute.
2. Add the ground lamb (or beef) to the skillet. Cook until browned, breaking it apart with a spoon (about 5-7 minutes).
3. Stir in tomato paste, Worcestershire sauce, broth, thyme, rosemary, salt, and pepper. Bring to a simmer and cook for about 5 minutes until slightly thickened. Stir in frozen peas and remove from heat.

Step 3: Assemble the Pie

1. Preheat your oven to 400°F (200°C).
2. In a baking dish, spread the meat and vegetable filling evenly across the bottom.
3. Spoon the mashed potatoes over the filling, spreading them out evenly. Use a fork to create texture on the surface (this helps with browning).

Step 4: Bake

1. Place the assembled Shepherd’s Pie in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and the filling is bubbling.
2. Let it cool for a few minutes before serving. Enjoy warm!