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.