Shepherd's pie is an outstanding use for leftover mashed potatoes. The addition of mushrooms, what a forager might find, makes for a delicious twist on the classic. This one-skillet supper also makes cleanup a breeze. If transporting or freezing the dish, place the beef mixture in a 9-inch square baking dish coated with cooking spray before adding the potatoes. We love meaty shiitake mushrooms here; cremini mushrooms would also work. Let the mashed potatoes, peas, and pearl onions thaw at…
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