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1
Prepare grill for medium-high heat. Set a wire rack inside of a roasting pan; pour in water to reach just below rack. Season fish inside and out with kosher salt and pepper. Stuff with onion, parsley sprigs, oregano, and 4 scallions. Set fish on rack and cover pan with foil, leaving one corner open. Place pan on grill and cook until steam escapes from vent, 3-5 minutes. Close vent and steam fish until cooked through, 15-20 minutes.
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2
Meanwhile, brush half of sliced lemon and half of sliced lime with 1 tablespoon olive oil and grill, turning once, until lightly charred, about 1 minute. Chop 2 slices of grilled lemon; set remaining slice aside for serving.
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3
Using a sharp knife, remove all peel and white pith from whole lemon and lime; discard. Working over a medium bowl, cut between membranes to release segments into bowl. Squeeze juice from membranes into bowl; discard membranes. Mix in chopped parsley, tarragon, red pepper flakes, sugar, chopped grilled lemon, remaining scallion, and remaining 3 tablespoons olive oil; season relish with salt and pepper.
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4
Arrange fresh and reserved grilled citrus slices on a platter; place fish on top.
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5
Drizzle with walnut oil, season with sea salt and pepper, and sprinkle with poppy seeds. Serve relish alongside.p
