Line a baking sheet with a lightly oiled piece of foil.
Remove skin and any bones from salmon, as necessary. Slice the salmon in half, doing your best to keep both halves symmetrical and even. This will help them layer with even thickness when the herbs are sandwiched between.
In a small bowl, add 2 Tbsp unsalted butter. Then add lemon zest, finely chopped basil and dill, and a sprinkle of salt & pepper. Mix the herb butter together until combined.
Pat the salmon filets dry with paper towels, then season lightly with salt and pepper. Spread mustard across one filet, and the herb butter over the other.
Sandwich two filets together, in opposite directions, so the resulting fish is of even thickness.
Roll out pastry thinly on a lightly floured surface, until ¼” thick. Place the salmon into the center, and wrap, making sure to encase the salmon fully. You may use the egg wash as a little extra "glue" to close up the seams.
Lightly score the pastry. I usually do 3 diagonal cuts across the top, making sure to break through the pastry to allow steam to vent.
Brush the pastry with beaten egg. Loosely cover and chill for 15 minutes.
Bake 20-25 minutes or until internal temperature is 145°F. Typically I remove from the oven at 140°F and allow residual temperature to bring it up to temp.