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1
Preheat the oven to 325 degrees F.
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2
Cut the squash through the equator to create 3 pieces: a 1-inch thick ring and 2 end pieces.
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3
Seed the squash.
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4
Wrap each end piece in foil.
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5
Wrap the onions in another piece of foil.
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6
Bake the squash and onions for 1 hour, then unwrap and allow to cool.
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7
Turn the oven up to 400 degrees F. Lightly grease 8 (4-ounce) ramekins or custard cups with butter or oil.
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8
Peel the cooled squash and cut the flesh into cubes.
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9
Pulse the cubes in the bowl of a food processor until pureed, or mash vigorously with a fork.
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10
Transfer the squash to a large bowl, add the egg yolk, eggs, Parmigiano-Reggiano, mascarpone, nutmeg, salt and pepper and stir gently to combine.
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11
Beat the egg whites to soft peaks, then gently fold them into the squash mixture.
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12
Divide the batter equally among the prepared cups, and place them in a baking pan large enough to hold them all without tipping.
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13
Fill the pan halfway with warm water, cover the whole pan tightly with aluminum foil, and bake for 35 to 40 minutes, or until a toothpick inserted in the center of the custard comes out clean.
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14
Keep warm.
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15
While the squash bakes, place a 12 inch-saute pan over medium heat and add 3 tablespoons oil.
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16
When the oil smokes add the shiitake and the sage and cook until soft and slightly crisp at the edges, about 8 minutes WITHOUT SHAKING THE PAN.
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17
Remove and allow to cool.
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18
Peel the reserved squash ring and cut the flesh into julienne strips.
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19
Place the squash in a small bowl, add 1 tablespoon of olive oil and salt and pepper, to taste, and toss well to coat.
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20
To make the vinaigrette: pour the vinegar into a small bowl.
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21
Gently whisk in remaining 1/2 cup olive oil and season with salt and pepper.
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22
To assemble the dish, carefully unmold 1 custard onto each of 8 warmed dinner plates, running a knife around the edge of the cup to loosen if necessary.
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23
Carefully halve the onions through the equator and place 1 half on each plate.
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24
Surround the custard with some of the mushroom mixture, garnish with the julienned squash and vinaigrette, and grate plenty of cheese over each plate before serving.
