Instructions: 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 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. (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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