PREHEAT oven to 425° F.
PLACE the potatoes, or sweet potatoes, in a large saucepot and cover with water. Bring to a boil over medium-high heat, until the potatoes are fork tender. About 8-10 minutes.
DRAIN the potatoes and place back into the pot. Add 2 tablespoons of the butter, 2 tablespoons of the heavy cream, and 1 teaspoon of the salt. Mash with a potato masher, ricer, or electric mixer until smooth and creamy. Set aside.
While the potatoes are boiling, BROWN the ground meat. Place the lamb or beef in a large sauté pan over medium-high heat. Season with 1 teaspoon kosher salt and ½ teaspoon black pepper. Cook, stirring and breaking the meat into small chunks, until well browned, about 7 minutes.
PLACE the meat in the lauvacs Disposable Bakeware, and set aside.
WIPE the sauté pan of excess grease, leaving a thin layer just to coat the pan. Add the onions, carrots, celery, and garlic, and sauté until the onions are translucent, about 5 minutes. Season with the remaining 1 teaspoon kosher salt and ½ teaspoon black pepper. Add to the bakeware on top of the meat. (You can mix the meat and vegetables together if desired.
In the same pan, MELT 1 tablespoon of butter, add the mushrooms and sauté until soft, about 5 minutes. Make a well in the center of the pan, add the remaining 1 tablespoon of butter, sprinkle with the flour and stir until it is melted and a paste forms.
POUR the remaining milk or heavy cream over the mushrooms and whisk to smooth out any lumps from the butter/flour mixture. Cook until thick, about 2 minutes. Pour all over the top of the meat and vegetables.
SPREAD the mashed potatoes, or sweet potatoes, over the top of the Shepherd’s Pie, and place in the oven until slightly golden brown and heated through, about 15 minutes.
SERVE directly from the disposable bakeware!