Shepherd's Pie is a cozy, old-fashioned casserole made from ground lamb, mashed potatoes and everyday vegetables. This layered meat and potatoes pie, is easy to make, especially with leftover mashed potatoes on hand.
Heat a large skillet over medium-low to medium heat. Add 1 tablespoon olive oil. Tilt the pan to coat the bottom. Add the 2 medium carrots, 2 stalks celery, 1 cup yellow onion, ½1 teaspoon kosher salt. Stir to combine and cover the skillet. Sweat the vegetables for 10-12 minutes until they're softened, but not browned.
Add ½1 teaspoon black pepper, 1½ teaspoons chopped fresh thyme, 1½ teaspoons chopped fresh rosemary, heat and stir for one minute until fragrant.
Increase the temperature to medium-high and add 1 pound ground lamb. Break the lamb apart with a wooden spoon or the back of a fork. Cook until the meat is browned and fragrant.
Drain all but 1 tablespoon of excess fat from the browned lamb.
Sprinkle with 1½ tablespoons all-purpose flour and cook, stirring constantly for one minute until the meat is coated and slightly pasty.
Stir in 1 teaspoon Worcestershire sauce, ¾ cup low-sodium beef broth, a pinch ground nutmeg, the remaining ½ teaspoon of kosher salt and black pepper. Cook and stir until the sauce thickens slightly into a gravy-like consistency.
Stir in 2 tablespoons chopped parsley, and ¾ cup frozen peas. Transfer the lamb mixture to a 1½ quart casserole dish and spread into an even layer.
Dollop 3 cups mashed potatoes over the lamb mixture and spread it evenly over the casserole with a spoon or spatula. Sprinkle the top with dots of butter and bake for 30-35 minutes until the shepherd's pie is heated through. Serve.