First, prepare your spinach filling. Place spinach in a bowl and add tomatos, feta and then toss with dressing. Set aside.
Next, roll out the puff pastry on a lightly floured surface. Cut it into 4 rectangles and work one at a time. Roll the quarter piece until it's thin. You'll have to eye it for size depending upon the size of your salmon.
Place about 3 heaping tablespoons in the center of the pastry lengthwise. You can use more if it will fit, leaving about 2 inches of pastry to bring up over the top. Place one salmon piece on top of the spinach mixture. Begin folding the ends and sides up over the filling...gently stretch the pastry tucking any stray spinach stems so as not to puncture the crust. Use water dampened fingers to make a sticky paste along the touching pastry edges to form a seal. Flip the pocket over and place seam side down on a cookie sheet lined with parchment paper. Repeat with all four portions. Line the pockets up about 2 inches apart on the sheet and puncture the top side with a knife in several places to vent. Brush the tops of the pockets with beaten egg and bake at 350 degrees until golden brown all over. Serves 4