Horseradish Mashed Potatoes With Caramelized Onions

From Bon Appetit, a nice side dish that can be made ahead. ZWT West region (Idaho potatoes).

• Ready In: 1 hr 20 mins
• Serves: 6

Ingredients

• 1⁄4 cup butter
• 2 1⁄4 lbs  white onions, thinly sliced
• 3  tablespoons  balsamic vinegar
• 2  teaspoons fresh thyme, chopped
• 3 1⁄2 lbs russet potatoes, peeled, cut into 1-inch pieces
• 7  tablespoons  whole milk
• 1⁄4 cup  prepared horseradish
• 6  tablespoons butter

Directions

1. Melt 1/4 cup butter in large skillet over medium heat. Add onions; saute until deep golden, about 30 minutes. Add vinegar and thyme. Reduce heat to low; saute 4 minutes. Season with salt and paper.
2. 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. Add 1 to 2 tablespoons more butter if necessary. Season with salt and pepper.
3. Transfer potatoes to serving dish. Spoon onion mixture atop potatoes. (Can be prepared 8 hours ahead. Cover with foil and refrigerate. Rewarm in 350ºF oven about 30 minutes). Serve hot.