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1
Melt butter in a large skillet over medium heat; saute shallots in the hot butter until translucent, 5 to 8 minutes. Stir mushrooms, salt, and black pepper into shallots. Turn heat to medium-high and cook, stirring often, until mushrooms are golden brown, about 10 minutes.
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2
Pour white wine over mushroom mixture, dissolving any browned bits of food on bottom of skillet into wine; bring to a simmer. Gently place scallops into wine and poach in the mushroom mixture until barely firm, about 2 minutes per side. Transfer scallops to a bowl. Strain mushroom mixture into another bowl, reserving mushrooms and cooking liquid separately. Return strained liquid to skillet, pour in any accumulated juices from scallops, and stir in cream. Bring to a boil and cook until cream sauce is reduced by about half, about 10 minutes. Stir often. Turn off heat and let mixture cool for 1 minute.
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3
Quickly whisk egg yolk into cream sauce until combined. Transfer skillet to a work surface (such as a heatproof countertop or cutting board) and stir cayenne pepper, 2 teaspoons tarragon, and lemon zest into sauce.
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4
Divide mushroom mixture into scallop shells, spreading mushrooms out to cover bottoms of shells; place about 3 scallops onto each portion. Spoon cream sauce over scallops to coat; let sauce drizzle down into mushrooms. Sprinkle lightly with Gruyere cheese and paprika or cayenne.
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5
Turn oven's broiler to high. Slightly crinkle a large sheet of aluminum foil and place onto a baking sheet. Place filled shells onto foil and press lightly to help them stay level.
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6
Broil about 10 inches from the heat source until sauce is bubbling and cheese is lightly browned, 5 to 6 minutes. Transfer to serving plates lined with napkins to prevent shells from tipping; garnish each portion with 2 crossed tarragon leaves.
