Shepherd's pie is a comforting, layered casserole made with a bed of ground lamb and vegetables in a tempting gravy topped with a layer of whipped potatoes.
Place cut potatoes in a large saucepan and cover with water. Bring to a boil, reduce heat to simmer and cook for 15 minutes or until potatoes are tender.
Meanwhile, heat one teaspoon of olive oil in a large skillet over medium-high heat. Add the mushrooms and a ¼ teaspoon of salt, stirring occasionally for 3-5 minutes until they are softened and give up their liquid. Transfer mushrooms to a large bowl.
Add one teaspoon of olive oil to the pan, stir in the onions, carrots, and another quarter teaspoon of salt. Cook over medium-high heat until onions are tender and translucent and carrots are softened, about 6-10. Taste the carrots to ensure they are tender and not crunchy. (If they are still crisp, turn the heat down to low and cover with a lid to help them steam. Transfer the vegetables to the bowl with mushrooms.
Heat the remaining teaspoon of olive oil and add the ground lamb, breaking apart the meat with the back of a fork and browning. When the meat is about halfway cooked, add the garlic, rosemary, oregano, ½ teaspoon kosher salt and ¼ teaspoon black pepper. Cook, stirring, until meat is fragrant and thoroughly cooked through. Transfer the lamb to the bowl with the vegetables and stir in the frozen peas.
Melt 1½ tablespoons butter in the pan and add the flour, ½ teaspoon salt and ¼ teaspoon black pepper. Stir to combine and cook over medium-high heat, stirring for about one minute, until bubbly.
Slowly add the beef broth and cook, stirring constantly, until the mixture thickens and comes to a boil. Boil and stir for one minute.
Add the Gravy Master (if using) and Worcestershire sauce. Remove from heat and add the gravy to the lamb and vegetable mixture. Stir to combine. Set aside.
Preheat the oven to 350°.
Heat the milk and two tablespoons of butter in the microwave or a saucepan until the butter melts and the milk is warm. Do not boil.
Drain the water from the potatoes. If using a potato ricer, select the smallest extruder disk and run the potatoes through the ricer, then stir in the milk and butter mixture, sour cream, salt, and pepper. Taste for seasonings and adjust to your liking. If using a potato smasher, add the milk and butter mixture, sour cream, one teaspoon of kosher salt and ½ teaspoon of black pepper to the potatoes. Smash the potatoes, incorporating the dairy products. Potatoes should be smooth but firm, holding their shape when you run a spoon through them.
Spray a large casserole dish with vegetable spray. Spoon the meat mixture into the casserole and spread out evenly with a spoon.
Spread the mashed potatoes over the casserole with a spatula, creating divots and swirls. (casserole can be made to this point, covered and refrigerated to bake later).
Add the remaining butter in small pieces over the top of the potatoes. Bake casserole in the center of a hot oven for 30 minutes, then turn on the broiler and cook for 5 minutes more until lightly browned.
Notes
Leftovers can be reheated in a 350° oven for 15 minutes.