In a large skillet over medium heat, melt 1 1/2 tablespoons butter.

Add mushrooms, half the shallots, half the salt, half the pepper, and half the thyme.

Cook, stirring frequently, until mushrooms are tender and almost completely dry, about 8 minutes.

Stir in spinach and cook until the mixture is very dry. Set aside to let cool.

Preheat the oven to 400°F.

On a well-floured surface, roll out pastry to a rectangle about 21 x 16 inches.

Use a sharp knife or pizza wheel to cut into 6 pieces, each about 7 x 8 inches.

Season salmon on all sides with the remaining salt and pepper.

Place a piece of salmon on each piece of puff pastry; if any ends of the salmon are particularly thin, tuck them under so the pieces are fairly uniform in thickness.

Top salmon with mushroom mixture.

Use your finger to moisten edges of pastry with a little water, then fold pastry sides in like a package and press to seal, completely covering the salmon and mushrooms.

Arrange seam-side down on a baking sheet lined with parchment paper and place in the freezer for 15 minutes.

Meanwhile, combine remaining shallot, thyme and wine in a small saucepan. Simmer until reduced to about 1/2 cup; set aside off the heat.

Brush the tops of the pastry with egg and bake in the top third of the oven until pastry is golden brown, about 25 minutes.

Reheat reduced wine; remove from heat and whisk in remaining butter a little at a time.

Spoon sauce onto plates and top with salmon.