Preheat oven to 400 degrees F. Place potatoes into a large saucepan, and cover with lightly salted water. Bring to a boil and cook for about 10 minutes, until just tender. Drain and reserve the potatoes.
Heat the butter or oil in a large cast-iron skillet. Add the onion and saute over medium heat for 7 minutes. Stir in the potatoes and corned beef. Add salt and pepper to taste, then mash the mixture down gently -- not too much -- with a potato masher or fork. Sprinkle the cream over the top. Leave on the heat for another minute or two, then transfer to the oven and bake for 30 minutes.
(To make Red-Flannel Hash, substitute 3 medium beets, cooked and diced, for 3 of the potatoes.)
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