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1
Preheat the oven to 350.
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2
In a pie plate, bake the walnuts for about 8 minutes, or until lightly browned, then let cool and coarsely chop.
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3
Line a medium baking sheet with parchment paper.
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4
Unfold the puff pastry and cut out four 4-inch rounds about 1/8 inch thick.
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5
Transfer the rounds to the prepared baking sheet and prick all over with a fork.
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6
Cover with another sheet of parchment paper and set a baking sheet on top.
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7
Bake the pastry for about 25 minutes, or until it is cooked and browned; remove the pastry from the oven.
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8
Increase the oven temperature to 400.
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9
Meanwhile, melt the butter in a large skillet.
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10
Add the onions and cook, stirring, over moderately high heat for 4 minutes; then cook over low heat, stirring often, until soft and golden, about 50 minutes; let cool.
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11
In a food processor, coarsely puree the onions and walnuts.
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12
Season with salt and pepper.
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13
In a medium saucepan of boiling water, blanch the mushrooms until just tender, about 2 minutes.
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14
Drain, pat dry and slice 1/4 inch thick.
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15
Spread the pastry rounds with the puree and arrange the mushrooms, overlapping, on top.
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16
Brush with olive oil and bake for 10 minutes, or until hot.
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17
Garnish with chives and serve.
