- 1/2 tablespoon unsalted butter, softened
- 1/2 teaspoon fleur de sel plus additional to taste
- 4 (6-oz) pieces center-cut salmon fillet (about 1 inch thick)
- 1/4 cup créme fraîche
- 2 tablespoons hot water
- 2 tablespoons avruga caviar (from a 120-g jar)
- 2 teaspoons olive oil
- 5 oz baby arugula (7 cups loosely packed)
- 5 oz mesclun (mixed baby salad greens; 7 cups loosely packed)
- 4 small lime wedges
- 1/2 teaspoon fresh lime juice
- Put oven rack in middle position and preheat oven to 250°F.
- 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.
- About 10 minutes before salmon is done, put créme fraîche 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 purée 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.
- 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.