Shallot Cream Sauce
In food processor, whirl cubed halibut with egg white until smooth. Blend in whipping cream, lemon juice, chervil, salt and pepper. Refrigerate in airtight container for at least
1 hour or for up to 4 hours.
Preheat oven to 400ºF (200ºC).
Cut salmon fillet crosswise into 1/2-inch (1 cm) thick slices; cover and refrigerate. Trim spinach, removing stems; rinse and shake off excess water. In large saucepan, cook spinach, with just the water clinging to leaves, just until wilted. Arrange in single layer on paper towels; pat dry.
On lightly floured surface, roll out puff pastry to form 14-inch (35 cm) square. Place on sheet of parchment paper large enough to cover large rimless baking sheet. Cover pastry with spinach, leaving about 1 1/2-inch (4 cm) border at edges. Spread half of the stuffing over spinach; layer with salmon slices, then remaining stuffing.
Mix egg yolk with 1 tablespoon (15 mL) water; brush some lightly on uncovered top and side borders of pastry. Starting at bottom edge and using paper as support, roll up, pressing egg-washed top edge onto pastry; fold in sides, pressing together lightly. Place roll and paper on rimmed baking sheet. (Make-ahead: Cover and refrigerate for up to 4 hours; add 10 minutes to baking time.) Brush lightly with egg wash. Bake in centre of oven for about 30 minutes or until golden and crisp.
To serve, trim off ends and cut pastry package crosswise into 8 slices. Pool one-quarter of the sauce onto each warmed plate. Arrange 2 slices per serving over and to side of sauce.
Meanwhile, in small saucepan, simmer shallots with vinegar over medium-low heat until vinegar is evaporated. Add cream; simmer until reduced by half, about 4 minutes. Whisking constantly, add butter, a few cubes at a time, to make creamy sauce. Add pepper.