Horseradish Mashed Potatoes with Caramelized Onions

1/4 c butter
6 tbsps butter
2 1/4 lbs vidalia onions -- thinly sliced
3 tbsps balsamic vinegar
2 tsps chopped fresh thyme
3 1/2 lbs russet potatoes -- peeled, cut in 1² pieces
6 tbsps whole milk -- to 8 tbsp
1/4 c horseradish

Melt 1/4 cup butter in large skillet over medium heat
Add onions; sauté until deep golden, about 30 minutes
Add vinegar and thyme.
Reduce heat to low; sauté 4 minutes.
Season with salt and pepper.
Meanwhile, cook potatoes in large pot of boiling salted water until very tender ; about 20 minutes. Drain.
Return to same pot; mash until smooth.
Add 6 tablespoons milk, horseradish and 6 tablespoons butter; stir to blend.
Thin with 2 tablespoons milk if necessary.
Season with salt and pepper.
Transfer potatoes to serving dish.
Spoon onion mixture atop potatoes. Or add a portion of onions and stir them in.(Can be prepared 8 hours ahead. Cover with foil and refrigerate. Rewarm in 350°F oven about 30 minutes.)
Serve hot.

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