To caramelize the onions, in a large skillet heat 1/4 cup of the butter. Add the onions and cook, stirring frequently, about 30 minutes or until golden brown. Season with the 1/2 teaspoon salt and pepper. Remove from the heat; set aside.

Meanwhile, cook the potatoes in a large pot, covered, in lightly salted boiling water for 12 minutes or until tender; drain.

Add the remaining 1/4 cup butter to the hot pot and let melt. Add the potatoes to the pot and mash with a potato masher until smooth. Stir in the milk, sour cream, and salt and pepper to taste. Stir in caramelized onions, reserving a few onions for garnish. Cook and stir over low heat until heated through. Transfer to a serving dish. Top with reserved onions. Makes 8 to 10 servings.

Prepare potatoes through step 3 stirring in the caramelized onions, but do not heat through. Transfer to a 2-1/2- to 3-quart casserole. Cover and chill in the refrigerator up to 24 hours or until ready to serve. Before serving, bake potato-onion mixture in a 325 degree F oven for 55 to 60 minutes or until heated through.