Before serving this elegant terrine, remove it from the refrigerator and let it sit for 20 minutes to take the chill off. The taste will be heightened, and it will cut cleanly into slices.
- 1 Tbsp. butter, softened
- 1 1⁄2 lb. skinless boneless whiting filets, coarsely ground in a food processor
- 1 1⁄2 cups crème fraîche
- 3 egg whites plus 1 egg yolk, at room temperature
- Salt and freshly ground white pepper
- 2-3 (1/2"-thick) skinless boneless salmon filets (1 1⁄4 lbs. in all)
- 1 Tbsp. dijon mustard
- 6 Tbsp. extra-virgin olive oil
- 2 tsp. fresh lemon juice
- 1⁄2 Tbsp. finely chopped chives
- 1⁄2 Tbsp. finely chopped flat-leaf parsley
Heat oven to 350°. Grease 2 strips of parchment paper all over with the butter: one measuring 4" × 16" and one 5" × 9". Line a 5" × 9" × 3" loaf pan with the 4" × 16" strip, allowing ends to drape over sides of pan. Stir together whiting, crème fraîche, 2 egg whites, and salt and pepper to taste in a bowl. Line bottom of loaf pan with half the salmon, skin side up. (Cut pieces to cover bottom entirely without overlapping.) Sprinkle with salt and pepper; top with half of whiting mixture, spreading it out to edges. Repeat process with remaining salmon (cutting to fit, if necessary) and whiting mixture to create 4 layers in all. Cover with remaining paper, then foil. Set loaf pan in a deep roasting pan and pour in enough boiling water that it reaches halfway up sides of loaf pan. Bake until just cooked through (a meat thermometer should read 120° when inserted in the center), 35–40 minutes. Set terrine on a plate; let cool for 30 minutes. Using a small weight, like an unopened soup can, press gently along top of terrine to compact it. Drain off any extracted liquid (you should lose only about 1⁄2 cup). Refrigerate overnight.
Whisk together egg yolk and mustard in a bowl. While continuing to whisk, drizzle in oil to form a thick mayonnaise. Stir in lemon juice. In a large bowl, whisk egg white until stiff peaks form. Stir one-third of egg white into mayonnaise, then gently fold in the rest. Season with salt and pepper to taste. Transfer to a serving dish.
Uncover terrine and loosen its edges with a knife; invert onto a large platter. Sprinkle with chives and parsley. Slice and serve with mayonnaise.