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1
Place potatoes in a large saucepan, and cover with cold water.
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2
Bring to a boil, reduce heat and cook at barest simmer until a skewer glides easily through potatoes, about 1 hour.
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3
Drain potatoes, cool slightly and peel.
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4
Run potatoes through a food mill and into a bowl.
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5
Grate onion on the large holes of a box grater directly into bowl with potatoes.
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6
Season with plenty of salt and some pepper.
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7
Stir in 1/2 cup creme fraiche.
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8
Cool to room temperature.
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9
(Potatoes can be covered and refrigerated for up to 1 day.)
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10
Set one wrapper on a work surface with one corner facing you.
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11
Spread scant 1/4 cup potatoes about 2 inches above bottom corner so that potatoes form 1-inch-wide strip that extends almost from one side of wrapper to the other.
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12
Fold bottom corner of wrapper over potatoes.
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13
Roll wrapper, folding in sides to enclose potatoes.
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14
When you get three-quarters of the way to the top, brush remaining edges with beaten egg and continue rolling to seal.
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15
Repeat with the rest of the wrappers and filling.
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16
(Knishes can be refrigerated, uncovered, for up to 1 day.)
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17
Heat 1 inch oil in a large, deep saucepan until very hot but not quite smoking, about 375 degrees.
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18
With tongs, add several knishes, and fry, turning once or twice, until golden brown all over, about 1 minute.
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19
Remove from oil, and drain on paper towels.
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20
Fry remaining knishes in batches.
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21
With serrated knife, trim a thin slice from either end of each knish.
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22
Cut each knish crosswise into three pieces.
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23
Arrange pieces, with one cut side up, on a platter.
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24
Top each piece with a dollop of remaining creme fraiche and a little caviar.
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25
Serve immediately.
