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1
Preheat the oven to 400F.
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2
On a floured surface, roll the puff pastry out to a 16-by-10-inch rectangle.
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3
Trim uneven edges.
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4
Place the pastry on a baking sheet.
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5
With a sharp knife, lightly score the dough to form a 1-inch border.
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6
Using a fork, prick the dough inside the border every 1/2 inch.
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7
Bake until golden, rotating the pan once, about 15 minutes.
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8
Meanwhile, in a small saucepan with a tight-fitting lid, toss the onion with 1 tablespoon of the oil.
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9
Season with salt.
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10
Cover and cook over medium heat until the onion begins to brown, about 5 minutes.
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11
Stir.
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12
Continue cooking with the cover on for 15 minutes, stirring every 5 minutes.
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13
Set aside.
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14
In a large saucepan with a tight-fitting lid, heat the remaining tablespoon of oil.
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15
Add the mushrooms; cover and cook until tender and all liquid has evaporated, about 10 minutes.
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16
Fold in the spinach; season with salt and pepper.
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17
Cover and cook until wilted, about 5 minutes more.
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18
Drain any liquid.
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19
Top the dough with the mushroom-spinach mixture.
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20
Scatter the onions and goat cheese on top.
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21
Bake until the cheese is lightly browned, about 15 minutes.
