Mix together the mustard, mayonnaise, lemon juice, dill, and goat cheese until smooth.
Slightly roll out the pastry and cut into eight square pieces.
Season the salmon with the salt and pepper and cut each filet into quarters.
Place one piece of salmon in the center of each piece of pastry.
Place 1/8 of the spinach on top of each piece of salmon and top with 1/8 of the cheese mixture.
Pull the pastry over the sides of the salmon and lightly seal the top. Seal the sides well with cold water. (This is so the top opens and looks pretty and the sides stay shut to hold everything inside.).
Bake on a lightly greased baking sheet for 30 minutes or until the pastry is golden.