Horseradish Mashed Potatoes with Caramelized Onions

Here is a way to change up an old standard.  These mashed potatoes aren't your Grandma's mashed potatoes.  They will get people's attention.  If you don't like horseradish, just leave that out and do the caramelized onion topping.  I like them either way.  This is a recipe that I got from a friend in South Carolina.  Where she got it I don't know, so can't really credit anyone beyond my friend. 

Ingredients:

1/4 Cup plus 6 TBSP. butter (11/4 sticks)
21/2 LBS. white onions - thinly sliced
3 TBSP. Balsamic Vinegar
2 Tsp. chopped fresh thyme
31/2 Lbs. russet potatoes, peeled and cut into one inch pieces
6-8 TBSP. whole milk or cream
1/4 cup prepared horseradish

Preparation:

Melt 1/4 cup butter in a large skillet over medium heat.  Add sliced onions; sauté until deep golden color, about thirty minutes.  Add vinegar and thyme.  Reduce heat to low and sauté another 4 minutes.  Season with salt and pepper. 

Meanwhile, cook potatoes in large pot of boiling salted water, until very tender, about twenty minutes.  Drain and return to same pot.  Mash until smooth.  Add 6 TBSP milk, the horseradish and 6 TBSP butter.  (I like to heat the butter and milk together in the microwave before adding them to the potatoes).  Stir together, or use a hand mixer to blend smoothly.  Thin with the two extra TBSP milk if necessary.  Season with salt and pepper.
Transfer the potatoes to a baking dish.  Spoon onion mixture over the top of the potatoes. (Can be prepared up to 8 hours ahead of time) Cover and refrigerate.  Rewarm in 350 degree oven for about thirty minutes before serving.  These go great with beef or ham.