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1
Heat the oven to 400 degrees F and arrange a rack in the top third.
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2
Melt 2 tablespoons of the butter in a large frying pan over medium heat.
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3
Add the onions and thyme, season generously with salt and pepper, and cook, stirring rarely and adjusting the heat as needed, until golden brown, about 35 minutes.
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4
Meanwhile, roast the mushrooms and toast the bread.
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5
Set a wire rack over a baking sheet.
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6
Clean the mushrooms and remove the stems (no need to scrape the gills).
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7
Brush both sides of the mushrooms with all of the oil and season generously with salt and pepper.
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8
Arrange the portobellos cap-side down on the rack.
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9
Roast the mushrooms for 8 minutes.
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10
Flip and roast until they just give way when pierced with a knife but still hold their shape, about 8 minutes more.
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11
Set the baking sheet aside to let the excess liquid drain from the mushrooms.
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12
Place the remaining tablespoon of butter in a medium microwave-safe bowl and heat until melted (alternatively, melt it in a small saucepan over low heat).
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13
Cut the bread into 1/2-inch cubes, add them to the bowl, season with salt and pepper, and toss to combine.
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14
Transfer the cubes to a second baking sheet and spread them in an even layer (reserve the bowlno need to wash it).
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15
Bake until toasted and golden brown, stirring halfway through, about 10 minutes total.
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16
Transfer the toasted bread to the reserved bowl and set it aside (reserve the baking sheetno need to wash it).
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17
Turn the oven to broil.
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18
When the onions are ready, add the sherry and cook, stirring occasionally, until completely evaporated, about 1 to 2 minutes.
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19
Remove from the heat, taste, and season with additional salt and pepper as needed.
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20
Place the mushrooms cap-side down on the reserved baking sheet.
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21
Divide the onions over the mushrooms.
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22
Divide the toasted bread over the onions and use your hands to gently press the bread into an even layer.
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23
Sprinkle the mushrooms evenly with the cheese.
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24
Broil until heated through and the cheese is melted and starting to brown in spots, about 2 minutes.
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25
Serve immediately.
