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1
In a small saucepan sprinkle gelatin over lemon juice and soften 10 minutes.
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2
In a bowl with an electric mixer beat together mascarpone and 1/2 cup sour cream until combined well.
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3
Quarter eggs and with back of a spoon force through a coarse sieve into mascarpone mixture.
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4
Add horseradish and stir well.
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5
Add remaining 1/4 cup sour cream to gelatin mixture and heat over moderately low heat, stirring, just until gelatin is dissolved.
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6
Add gelatin mixture to mascarpone mixture and stir well.
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7
Cut about one third salmon into strips and arrange 1 in every other ridge of a 3-cup brioche or other mold, letting ends hang over edge.
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8
Chop remaining salmon into 1/2-inch pieces and fold with chives into mascarpone mixture.
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9
Spoon mousse into mold and fold ends of salmon strips over top.
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10
Chill mousse, covered, at least 2 hours, or until firm, and up to 2 days.
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11
Run a thin knife along inside of each ridge of mold and dip mold into a bowl of hot water a few seconds.
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12
Invert a plate over mold and invert mousse onto it.
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13
With a slotted spoon arrange pickled onions decoratively on and around mousse touching each other, and keeping covered with plastic wrap.
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14
Brush mushroom rolls all over with melted butter and rearrange, seam sides down and barely touching, in pan.
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15
Chill mushroom rolls, covered, at least 1 hour and up to 1 day.
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16
Preheat oven to 425F.
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17
Bake mushroom rolls in upper third of oven 15 minutes, or until golden.
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18
Halve rolls crosswise and tie a chive around each half.
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19
In a saucepan barely cover onions with cold water.
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20
Bring water to a boil and simmer 1 minute.
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21
Drain onions in a colander and return to pan with vinegar, salt, and cold water to just cover.
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22
Bring liquid to a boil and simmer 3 minutes.
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23
Remove pan from heat and cool onions in liquid.
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24
Pickled onions may be made 1 week ahead and chilled in liquid, covered.
