Blend half the salmon with salt and double cream in a food processor until smooth.
Fold in white crab meat, herbs, and citrus zests, then shape into four 100g sausages.
Cut pastry into four 17x15cm rectangles and place salmon sausages lengthways on each.
Brush edges with egg wash, fold pastry to encase filling, and seal edges with a fork.
Trim excess pastry, apply two coats of egg wash, and chill until ready.
Preheat oven to 200°C and line a tray with baking paper.
Bake pastries for 11 minutes until golden-brown and puffed.
Dissolve soaked gelatine in lemon juice over low heat, then transfer to a blender.
Add brown crab, condiments, and egg yolk to the blender and process until smooth.
Add breadcrumbs and blend again before slowly drizzling in the oils while mixing.
Season to taste and pass the ketchup through a fine sieve.
Serve warm rolls with the crab ketchup on the side.