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Melt 3 tablespoons of butter in a deep skillet and add the onion. Cook for 10 minutes until soft and season with salt. Lower heat and continue cooking, stirring occasionally for 40 minutes to 1 hour or until onion has a deep caramel color.
While the onion is cooking, place potatoes in a large pot filled with water. Add salt to the water and bring to a boil. Cook potatoes until tender, drain and peel. Mash potatoes or force them through a ricer and transfer to a saucepan. Add hot milk and stir to make a thick puree.
Process the caramelized onions in a food processor until smooth and stir into the mashed potatoes. Add remaining butter and parmesan cheese and stir. Stir in milk until the desired consistency. Correct seasonings. |