Preheat oven to 400 degrees F (200 degrees C.) Place spinach in a steamer over 1 inch of boiling water, and cover. Cook until tender, about 2 to 6 minutes. Drain and cool..
Place spinach in the bottom of a 2 quart casserole dish. Cover with chopped chicken. Sprinkle on the sliced mushrooms and season with salt and pepper. Spread sour cream evenly over top. Cover with puff pastry.
Prick the puff pastry in several places to allow steam to escape. Brush top with beaten egg. Bake in the preheated oven for 20 minutes, or until golden brown.