Place the butter in a frying pan and gently cook on a medium low heat the onion and garlic until the onion becomes translucent. Add the cream cheese, breadcrumbs and parmesan, mix together well then add in the spinach cooking for 5 minutes. Meanwhile, season the salmon fillets with salt and pepper and brush with mustard. Unroll the puff pastry sheets and cut into 4 equal pieces. Place the salmon (skin up) on each piece of pastry and spoon the spinach mixture on top. Mix the egg yolk with a little water and brush around the edges of the pastry before wrapping up tightly into parcels. Place the parcels onto baking sheets lined with non-stick baking paper and brush with the remaining egg wash. Lightly cut lines on the pastry with a knife for decoration but do not pierce the pastry. Bake in a medium oven for about 30 minutes or until well browned and puffed. Serve with mashed potatoes and fresh lemon slices.