While the puff pastry is baking, cut the salmon into escalopes: working from tail to head, run your fingers along the surface of the salmon fillets to detect any small bones; remove them. Holding the knife at a shallow angle to the fish, cut the salmon into 6 equal portions, or escalopes. Set the salmon aside.
Stem the spinach and rinse the leaves in several changes of water, then coarsely chop it.
Melt 1 oz of the butter in a saucepan over medium heat, add the spinach, and cook, stirring frequently, until the moisture has evaporated. Season to taste with salt and pepper; cover to keep warm.
Snap off and discard the tough ends of the asparagus. Starting 1 inch below the tip, peel each stalk with a vegetable peeler. Tie the asparagus into several even bundles with fine string. Bring a large saucepan of salted water to the boil. Add the asparagus and cook over high heat for 10-12 minutes, until tender. Drain and cover to keep warm.
Prepare the lemon buttersauce: cut 7 oz butter into small pieces. Bring the lemon juice to the boil in a small saucepan and reduce by half. Whisk in the butter little by little, until the sauce is creamy. Season to taste with salt and white pepper; keep warm off the heat in a saucepan of hot water.
Cook the salmon: season both sides of each escalope with salt and pepper. Heat 0.5 oz butter in a large frying pan over medium heat. Add the salmon and sauté for 2-3 minutes on each side, until lightly cooked.
To serve, cut each pastry rectangle in half horizontally. Place the bases on 6 serving plates. Make a bed of spinach on each pastry base and top with a piece of salmon. Arrange the asparagus spears on the plates. Pour the hot lemonbutter over the salmon and asparagus. Sprinkle with chives. Top with the remaining puff pastry halves, tipped slightly backwards to resemble the half-open lid of a box. Serve immediately.