Preheat the Oven: Set your oven to 400°F (200°C) so it’s hot and ready for the casserole.
Brown the Meat: In a large skillet over medium heat, add the ground beef or lamb. Cook, breaking it up with a spoon, until browned all over (approximately 6-8 minutes). Drain any excess fat to keep the dish from being greasy.
Sauté the Aromatics: Add the diced onion and minced garlic to the skillet. Sauté for about 3-4 minutes until the onion is soft and translucent.
Add the Vegetables: Stir in the diced carrots and continue cooking for 5 minutes until they start to soften.
Combine the Ingredients: Add the frozen peas to the mixture, along with the beef broth, tomato paste, Worcestershire sauce, dried thyme, dried rosemary, salt, and pepper. Stir well to combine and bring to a gentle simmer. Allow to cook for 5-7 minutes, stirring occasionally, until the sauce thickens slightly and flavors meld together.
Prepare the Casserole Dish: Transfer the hearty meat and vegetable mixture into a greased 9x13 inch casserole dish, spreading it out evenly across the base.
Top with Mashed Potatoes: Spoon an even layer of prepared mashed potatoes over the meat mixture. Use a spatula to smooth it out. If you like a cheesy topping, sprinkle the shredded cheddar cheese evenly over the mashed potatoes.
Bake the Casserole: Place the casserole in your preheated oven. Bake for 25-30 minutes, or until the top is golden brown and the edges are bubbling, indicating it’s heated throughout.
Cool and Serve: Once baked, remove the casserole from the oven and let it sit for 5-10 minutes to cool slightly. Garnish with freshly chopped parsley for a burst of color and freshness before serving.
Notes
Scoop the Shepherd's Pie onto plates or bowls. A sprinkle of extra parsley on top adds a beautiful finishing touch. Consider pairing it with a simple side salad for a complete meal!