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1
Preheat the oven to 375.
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2
Line a large rimmed baking sheet with parchment paper.
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3
In a very large bowl, thoroughly blend the egg whites and kosher salt.
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4
On the baking sheet, form part of the salt mixture into a rectangular bed 1 inch larger than the piece of salmon and 1 inch deep.
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5
Set the salmon in the center of the salt mixture and season it inside with sea salt and pepper.
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6
Pack the remaining salt around it, pressing the salt against the fish so it adheres.
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7
Bake the salmon for about 50 minutes, or until an instant-read thermometer inserted through the salt crust into the thickest part of the fish registers 125 if you like a rare streak, or 130 for more thoroughly cooked fish.
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8
Remove the salmon from the oven and let it rest for 15 minutes.
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9
Meanwhile, in a small saucepan, simmer the Prosecco and scallion whites over moderately high heat until the liquid is reduced to 2 tablespoons, about 15 minutes.
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10
Reduce the heat to low and whisk in the butter, 1 chunk at a time, making sure it is fully incorporated before adding more; do not boil.
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11
Stir in the scallion greens and season with sea salt and pepper.
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12
Using the back of a serrated knife, knock along the edge of the salt crust to crack it, then insert the knife blade to loosen the crust; carefully lift off the pieces.
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13
With 2 large forks, peel off the salmon skin and scrape off the dark flesh.
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14
Lift the fish from the bones and transfer to plates.
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15
Spoon the sauce over the salmon and serve.
