Put the onions in the bowl with the fat or butter. Cover the bowl and microwave for 10 minutes on high.
Take off the cover, and stir the onions--there should be good bit of liquid in the bowl. Microwave (uncovered) for approximately 10 more minutes, but keep an eye on it. It should get brown around the edges, so stir it periodically.
Eventually it'll be a brown mass of onions. If you like them crispy, microwave them a bit longer but, again, keep an eye on them--it's a real short moment between crispy and burnt.
You can do this in a conventional oven too, but it takes longer; say around 20 minutes at 350°F with the cover on and another 20 min with the cover off, stirring periodically. Once again, keep an eye on it.