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1
Heat the oven to 375 degrees F and arrange a rack in the middle.
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2
Coat a shallow 2-quart baking dish with butter; set aside.
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3
Place the porcinis in a medium heatproof bowl.
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4
Pour the boiling water over them and let the mushrooms sit until softened, about 12 minutes.
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5
Meanwhile, spread the bread cubes in an even layer on a baking sheet and bake until light golden brown, about 10 minutes.
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6
Transfer to a large bowl and set aside.
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7
Heat the measured butter and oil in a large frying pan over medium-high heat until the butter is foaming.
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8
Add the chanterelle and cremini mushrooms and cook, stirring rarely, until the mushrooms have released their liquid and are golden brown, about 8 to 10 minutes.
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9
Add the shallot and thyme, stir to combine, and cook for 1 minute.
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10
Using a slotted spoon, transfer the rehydrated porcinis to the pan (be careful not to disturb the gritty sediment at the bottom of the bowl) and set the mushroom liquid aside.
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11
Cook, stirring occasionally, until the shallots have softened, about 1 minute more.
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12
Add the wine, scrape up any browned bits from the bottom of the pan, and cook until the wine has almost completely evaporated, about 1 to 2 minutes.
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13
Taste and season with salt and pepper as needed.
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14
Scrape the mushroom mixture into the bowl with the toasted bread cubes (reserve the pan), add the egg, and stir to combine.
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15
Measure 1/2 cup of the reserved mushroom liquid, being careful not to include the sediment, and discard the remaining liquid.
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16
Drizzle the measured liquid into the bowl and stir to combine.
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17
Transfer the mushroom-bread mixture to the prepared baking dish and spread it into an even layer; set aside.
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18
Return the reserved pan to medium-low heat, add the cream, and bring to a simmer.
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19
Whisking constantly, add the cheese 1 piece at a time, letting each piece melt before adding the next.
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20
(If you add the cheese too quickly or stop whisking, the sauce may break.
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21
If it does break, sprinkle in cornstarch 1/2 teaspoon at a time and add more cream a splash at a time while whisking until the sauce is bubbling, creamy, and smooth again.)
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22
Taste and season with salt and pepper as needed.
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23
Pour the cheese sauce evenly over the mushrooms and bread.
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24
Bake until browned and bubbling, about 15 to 20 minutes.
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25
Serve immediately.
