Step 1: Prepare the potatoes. Place the peeled and quartered potatoes in a large pot, cover with cold salted water, and bring to a boil. Reduce heat and simmer until fork-tender, about 15-20 minutes. Drain thoroughly.
Step 2: While potatoes cook, start the meat filling. Heat olive oil in a large oven-safe skillet or Dutch oven over medium-high heat. Add the ground lamb and cook, breaking it up with a spoon, until browned. Drain off any excess fat. Transfer the browned lamb to a bowl and set aside.
Step 3: In the same skillet, add the chopped onion, carrots, and celery. Cook over medium heat, stirring occasionally, until softened, about 8-10 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 4: Sprinkle the flour over the vegetables and stir for 1 minute to cook out the raw flour taste. Pour in the beef/lamb broth and red wine (if using), scraping up any browned bits from the bottom of the pan. Stir in the tomato paste, Worcestershire sauce, thyme, and rosemary. Bring to a simmer.
Step 5: Return the cooked lamb to the skillet. Add the frozen peas. Season generously with salt and pepper. Let the mixture simmer gently for 5-7 minutes, allowing the sauce to thicken and the flavors to meld. If the sauce becomes too thick, add a splash more broth. Transfer the filling to a 9×13 inch baking dish if not using an oven-safe skillet.
Step 6: Finish the mashed potatoes. While the filling simmers, mash the drained potatoes with a potato masher or ricer. Heat the milk and butter together until butter is melted. Add the warm milk and butter mixture to the mashed potatoes and mix until smooth and creamy. Season with salt and pepper to taste.
Step 7: Assemble and bake. Preheat your oven to 375°F (190°C). Spoon the mashed potatoes evenly over the lamb filling, spreading them to the edges. You can create decorative ridges with a fork for a nice golden crust. For easier serving and storage, consider using Basics Glass Food Storage dishes.
Step 8: Bake for 25-30 minutes, or until the filling is bubbling around the edges and the potato topping is golden brown. If you want a deeper golden crust, you can briefly broil for the last few minutes, watching carefully to prevent burning. Let the shepherds pie recipe rest for 10-15 minutes before serving to allow the filling to set.