If the salmon skin was not previously removed, make a small slice right under the skin and peel the skin off. Cut the filets in half if needed to fit a dimension of about 4 inches wide by 8 inches tall. Generously salt with truffle salt and add some fresh grated pepper. Set aside.
Grate the lemon rind. Mince the herbs. Place all in a bowl with the softened butter and muddle together until into a soft, spreadable consistency.
Lay the puff pastry out on a floured pastry sheet and roll to about 1/8 thickness. With a sharp knife cut even, diagonal slits one third of the way into the crust on both sides. Keep same number of strips on each side. The bottom two strips can be flipped over to create the "tail."
Slather the mustard down the middle of the pastry sheet and place salmon filets on top.
Smear the butter-herb mixture over the filets. Alternating from one side to the other, bring each strip diagonally across the salmon filets. Cut out a hole for his eyeball, if so desired. Or be creative with olives.
Mix the egg with a little water and brush over the pastry.
Bake the salmon for 20–25 minutes, until the pastry is golden brown and crisp. To test if ready, insert a skewer into the middle. It should feel warm for perfectly cooked salmon.
Remove from oven and let rest for 5 minutes before carefully slicing with a serrated knife into serving size pieces.