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1
Preheat oven to 375F.
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2
Trim root ends flush with onions (so they will stand upright).
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3
Cut off tops of onions 1 inch from pointed blossom end and reserve.
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4
In a small flameproof baking pan stand onions upright and replace reserved tops.
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5
Brush onions with oil a roast in middle of oven 1 hour and 15 minutes, or until centers are just knife-tender.
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6
While onions are roasting, in a saucepan boil potatoes in water to cover 15 minutes, or until tender, and drain in a colander.
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7
Cool potatoes until they can just be handled and peel.
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8
Return potatoes to pan.
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9
Transfer onions to a work surface to cool and add wine, water, and cream to baking pan.
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10
On top of stove cook cream mixture over moderate heat, stirring, until brown bits scraped from bottom of baking pan are just dissolved and add to potatoes.
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11
Leaving outermost layer of each onion inside skin, carefully scoop remaining layers out of onion shells with a small spoon and chop.
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12
In a small skillet saute chopped onion in butter over moderately high heat until golden and add to potato mixture.
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13
Increase oven temperature to 425F.
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14
Mash potato mixture with a potato masher until potatoes ate coarsely mashed and liquid is incorporated and force through a food mill fitted with fine disk into a bowl.
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15
Stir in Parmesan and season with salt and pepper.
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16
Spoon potato mixture into onion shells and replace tops.
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17
Onions may be prepared up to this point 1 day ahead and chilled, covered.
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18
In a small baking pan roast onions in middle of oven until heated through, 15 to 25 minutes.
