Cook the Ground Meat:
In a large skillet, cook the ground lamb (or beef) over medium heat, breaking it up with a spoon as it cooks. Once browned, drain any excess fat, if necessary.
Sauté Vegetables:
Add the diced onion, carrots, and garlic to the skillet with the meat. Cook for about 5 minutes, or until the vegetables begin to soften.
Add Broth and Seasonings:
Stir in the tomato paste, Worcestershire sauce, thyme, salt, and pepper. Pour in the beef broth and bring the mixture to a simmer. Cook for about 10 minutes, until the sauce has thickened slightly. Add the peas and stir until they’re evenly distributed. Remove from heat.
Prepare Mashed Potatoes:
While the filling is cooking, prepare the mashed potatoes. If they’re not already prepared, boil peeled and diced potatoes until tender (about 15 minutes), then mash them with butter, milk, salt, and pepper until smooth and creamy.
Assemble the Shepherd's Pie:
Transfer the meat and vegetable mixture into a 9x13-inch casserole dish or another baking dish. Evenly spread the mashed potatoes on top of the meat mixture, smoothing it out with a spatula. If desired, sprinkle grated cheddar cheese over the top for extra flavor and a golden finish.
Bake the Pie:
Preheat the oven to 400°F (200°C). Bake the Shepherd's Pie for 20-25 minutes, or until the top is golden brown and slightly crispy.
Serve:
Allow the Shepherd’s Pie to rest for a few minutes before serving. Scoop into portions and enjoy!