Boil or steam the potatoes until tender and mashable. Drain. Let the
poratoes rest for a few minutes. Coarsely mash with a fork, leaving a
few irregular chunks. Preheat the broiler. Halve the onions
lengthwise; cut each half into thin half-moon rounds. In a large
skillet, melt 1/3 of the butter over medium-high heat. Add the onions
and saute until soft and golden, about 10 minutes. Add 1/3 more of
the butter and melt. Add the potatoes, season generously with nutmeg,
salt and pepper, and cook, tossing the mixture about 2 or 3 more
minutes. Transfer the mixture to a gratin dish. Smooth with the back
of a large spoon, then dot with the remaining butter. Brown lightly
under the broiler. Serve immediately.
