Mashed Potatoes with Caramelized Onions

I served this with a prime rib roast and some vegetable (sorry, I don't remember, so shoot me).  Anyway, these potatoes are tremendous.  It takes a little time to make the onions, but they are so  gouda (My favorite puns are cheese puns). 

First the onions.  I don't know how many people you're cooking for but since onions cook down so much, you might consider having extras.  Not a bad idea matey. 

Start with 1/4 cup butter and 6 sweet onions.  Peel onions and slice into very thin slices.  Melt the butter in a large sauté pan over med heat.  Add the onions and cook stirring SELDOM.  As the onions begin to brown, you will need to be more vigilant in stirring.  After 15 minutes or so, you should see the onions start to nicely caramelize.  Be sure to stir frequently so they don't burn.  You want them a nice brown color.  It should take about 25 minutes.  Add a tablespoon of good balsamic vinegar and salt and pepper to taste.  Keep warm or if you made ahead, reheat.  Make your potatoes.  I use Yukon Gold for mashed potatoes.  They are just so much more creamy.  Don't tell me I have to tell you how to cook potatoes in boiling water?  When they're done, strain and then use a ricer to get really creamy potatoes.  Once they're thru the ricer, add some unsalted butter and add hot cream until you get the consistency you would want.  It's not potato soup, you know?  Add salt and pepper to taste.  Plate the potatoes and put the onions on top of the potatoes.  That is some good cookin'!