Preheat the oven to 180C.
Heat the butter in a non-stick frying pan and pan fry the spinach leaves until they wilt – about a minute.
Put the spinach in a sieve and allow some of the liquid to drain away.
Mix the cream cheese, spinach, garlic and chives in a bowl until combined. Season with salt and pepper.
Pat the salmon fillets dry with kitchen paper and then divide the cheese mixture over the top of the four salmon fillets. Chill in the fridge for 10 minutes.
Unroll the puff pastry, leaving it on the paper it comes rolled in.
Cut the pastry in half and then put one salmon fillet on one side of each rectangle.
Brush a little beaten egg around the edge of the pastry and fold over to enclose the fish.
Trim away any excess pastry and crimp the edges with a fork to seal.
Brush with the beaten egg and use any spare pastry to cut out decorative shapes (optional).
Repeat with the other two salmon fillets and place the salmon en croûte on a heavy baking tray lined with baking paper.
Cook for 20-25 minutes until the pastry is golden. Serve with steamed greens.