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.