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1
Put oven rack in middle position and preheat oven to 250u00b0F.
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2
Spread butter over bottom of a 13- by 9-inch baking dish and sprinkle with 1/2 teaspoon fleur de sel. Put salmon, skin sides down, in dish and bake, uncovered, until just cooked through, 30 to 35 minutes.
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3
About 10 minutes before salmon is done, put creme fraiche in a 1- to 1 1/2-quart heavy saucepan and heat over low heat, stirring occasionally, until just melted. Transfer to a blender along with water and caviar and puree into a very smooth emulsion. Return to saucepan and keep warm, uncovered, over very low heat, stirring occasionally. Thin sauce slightly with a little additional hot water if too thick.
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4
Just before salmon is done baking, heat oil in a 12-inch skillet over moderately high heat until hot but not smoking, then add greens in 3 batches, stirring and turning with tongs until greens are slightly wilted before adding each new batch, and continue to cook, stirring, until completely wilted, 1 to 2 minutes. Push greens to one side of skillet and pour off liquid (discarding it). Divide greens among 4 plates. Top greens with salmon and season lightly with fleur de sel, then squeeze some of lime juice from wedges over salmon. Stir 1/2 teaspoon lime juice into sauce and spoon onto plates.
