5 Mash potatoes in bowl with remainder of butter, season to taste.
6 Place beef and onions in baking dish. Distribute mashed potatoes on top. Rough up with a fork so that there are peaks that will brown nicely. You can use the fork to make some designs in the potatoes as well.
7 Cook in 400 degree oven until bubbling and brown (about 30 minutes). Broil for last few minutes if necessary to brown.