Preheat the oven to 425 degrees. Place puff pastry rectangles on a baking sheet lined with parchment. Brush surface of each piece with beaten egg without letting egg drip down sides of pastry. Bake for 10 minutes. Lower heat to 350 degrees and bake 25 minutes longer or until pastry is nut brown and crisp to the touch.
Meanwhile melt 6 tablespoons of butter over medium heat. Saute shallots for a minute. Add onion and saute 5 to 6 minutes longer or until tender. Add apples and cook for 15 minutes or until very tender. Season well with salt and pepper; set aside until later.
Remove pastry from the oven. Split each piece in half by carefully pulling the top piece off the bottom with your fingers. Do this with a light touch or the fragile pastry will break apart. Season each piece of chicken with salt and pepper.
Melt remaining 2 tablespoons of butter in a 12-inch skillet. Saute chicken over fairly high heat for 3 minutes a side. Remove pan from the heat.
Quickly reheat the apples. When hot, add the spinach and heat just until wilted. Season with the vinegar, salt and pepper.
Place bottom half of pastry on a dinner plate. Top with chicken and spoon spinach and apple mixture, with juice over the top. Lay top piece of pastry, slightly askew across the spinach. Serve immediately.