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1
Place a rack about 4 inches from the broiler element and preheat.
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2
Put the potato in a medium saucepan, cover with water by 2 inches, and season generously with salt.
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3
Bring to a boil over high heat, lower the heat, and simmer until very tender, about 10 minutes.
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4
Drain and cool slightly.
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5
Meanwhile, preheat a large skillet over medium-high heat.
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6
Add 2 tablespoons of the butter and heat.
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7
Season the salmon all over with salt and pepper.
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8
When the butter's foaming subsides, place the salmon, in the skillet and cook, turning once, until well-browned, about 1 1/2 minutes per side.
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9
Transfer to a baking sheet.
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10
Put the cooked potatoes in a small bowl with the 2 teaspoons salt, 2 tablespoons butter, half-and-half, and horseradish and, using a fork, mash until fluffy.
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11
Season with pepper, to taste.
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12
Melt the remaining 2 tablespoons butter in a small saucepan.
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13
Using your hands, press about 1/4 of the potato mixture onto the top of each salmon fillet, to form a thin crust.
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14
Lightly brush the potato with the melted butter.
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15
Place the salmon under the broiler and cook until still slightly pinkish inside and the crusts are golden brown, about 10 minutes.
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16
Transfer to serving platter and garnish with the chive segments.
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17
Serve immediately.
