Directions

• Cut thickest portion of salmon loin into a rectangle 8 inches long and 3 inches wide; trim and remove thin section of flesh. You will have a 14-ounce piece of salmon left; reserve trimmed salmon for another use.
• Bring a medium pot of water to a boil. Stir in 1 tablespoon salt. Prepare a large bowl of ice water. Add cabbage leaves to boiling water, pressing to submerge. Cook until wilted and pliable, about 2 1/2 minutes. Transfer leaves to ice water to cool. When leaves have cooled, transfer to a baking sheet covered with a few layers of paper towels to drain, shaking off excess water. Remove 2 to 3 inches of the thick stalks on bottom of each leaf by cutting on either side to remove. Pat cabbage dry.
• Place shrimp, egg whites, and fish sauce in a food processor and blend just until smooth. Transfer shrimp mousse to a medium bowl and fold in chives, dill, and lemon zest.
• Season salmon with salt. Place a sheet of nori on a work surface. Lightly dampen hands and pat nori to dampen very slightly. Place salmon on top of nori (nori will begin to shrivel once dampened). Quickly wrap salmon with nori; if it doesn’t completely cover salmon, cut a piece from a second sheet of nori to fit.
• Place a large sheet of plastic wrap on a work surface. Arrange cabbage on plastic, forming an 8- x 8-inch square completely covered with an even layer of cabbage. Using an offset spatula or spoon, spread shrimp mousse evenly on top of cabbage to a thickness of 3/4 inch. Place salmon in center. Use kitchen scissors if needed to trim cabbage to match the length of the salmon. Roll cabbage around salmon, using plastic wrap overhang as an aid while rolling. Wrap salmon roll in 5 layers of plastic wrap, forming a firm, tight cylinder. Freeze 30 minutes.
• Place another large sheet of plastic wrap on a work surface. Place 1 sheet of puff pastry on top. Roll out slightly into a 9-inch square. Lightly brush pastry with egg yolks. Remove plastic wrap from salmon roll and place salmon in center of pastry. Roll the pastry over, pressing seam to seal; trim off any excess pastry and press ends to seal. Roll Wellington tightly in plastic wrap and freeze 30 more minutes. Cover and refrigerate remaining egg yolks.
• Preheat oven to 425°F. Unwrap Wellington and place, seam side down, on a baking sheet lined with parchment paper. Chill in refrigerator 30 minutes.
• Cut remaining piece of puff pastry with a plastic lattice cutter and then place it evenly on the Wellington, or cut into 1/2-inch strips and arrange in a lattice, trimming edges to fit Wellington. Brush with beaten egg yolks and place lemon thyme leaves in every gap of the lattice. Sprinkle with flaky salt.
• Transfer Wellington to a fresh sheet of parchment paper and return to baking sheet. Using a sharp knife, slash top of pastry in a few spots. Bake until pastry is dark golden brown and an instant read thermometer registers 125°F, about 45 minutes, tenting with foil if browning too quickly after 25 minutes cooking. Allow Wellington to rest for 30 minutes before slicing.