Preheat the oven to 400Â°F.
Season tenderloin all over with salt and freshly ground black pepper. Heat a large skillet or roasting dish and add 2 counts (approximately 2 tbsp) of olive oil. Sear the tenderloin until it has a nice deep brown color all over it and then put it into the pre-heated oven to bake for 20-25 minutes or until medium and juices run pink.
Prepare the pastry by laying it out flat on a sheet tray lined with parchment paper. Using a pastry cutter, cut the sheet into 4x4 inch squares and separate slightly. Brush the pastry all over with the melted butter and then sprinkle liberally with the chopped rosemary and some coarse sea salt. Use a fork to pierce the pastry so it doesn't rise too much when cooking. Bake in the oven for 10-12 minutes or until slightly puffy and golden.
Meanwhile prepare the spinach. Heat the olive oil and butter over high heat and then add the garlic. Add the spinach and then the nutmeg and cook until just wilted. Season with salt and pepper and then set aside. In a separate pan, cook mushrooms over high heat with chopped garlic, butter and thyme. Season with salt and pepper, set aside.
Once the tenderloin is cooked remove from the oven and set the tenderloin aside to rest. Cover loosely with tented foil. Set the pan on a stove top and dust with flour. Deglaze (add) with wine and the stock scraping the bottom of the pan with a wooden spoon to extract all the flavors. Reduce over medium heat until thick and syrupy.
Take puff pastry square and slice horizontally in half. To assemble lay bottom half of the puff pastry square on the plate. Heap a spoonful of spinach on one side of square, a serving of mushrooms on the other and top with a couple of thick slices of tenderloin. Spoon over the remaining tenderloin juice to cover the meat and drizzle around the plate. Finish with other slice of puff pastry.