Preheat the oven to 400ºF. Line the bottom of a 9x13” casserole dish with parchment paper. Place one of the halves of the squash in the casserole and cover it with foil. Reserve the other half for another purpose. Roast for about 25 minutes or until the squash can be easily pierced with a fork. Remove the foil and set aside.

Pulse mushrooms and onion in a food processor into a fine mince. Depending on the size of your processor, you may need to do this in two batches. Alternatively, mince the mushrooms and onion by hand. Heat the oil in a large pan over medium-high heat. Add the mushrooms and onion to the pan. Cook for a few minutes then stir in the soy sauce, thyme, and pepper. Cook, while stirring occasionally, for about 20 minutes or until most of the moisture has evaporated. Stir in the lemon juice. Cook for another couple of minutes. Mushrooms should have a moist, spreadable consistency, but not be too wet or too dry. Remove from heat and set aside.

Lay a piece of parchment cut to the size of a small baking sheet on a large work surface. Dust it with flour. Unfold the puff pastry sheet so that the longest side is horizontal to the edge of the counter. Roll it out so it’s about 1 inch larger on all sides. Spoon the mushrooms onto the pastry and spread evenly, leaving about a ½-inch border. Place the butternut squash, cut-side down, onto the horizontal center third of the pastry. Using the parchment paper to help lift the puff pastry, carefully and quickly lift the parchment along the long sides up and over the squash to cover. Pinch the edges of the dough to seal. Tuck in the ends of the dough. Brush the pastry with a little oil and sprinkle with salt. Score the dough several times to allow for steam to escape. Lift the parchment onto the baking sheet and place it in the oven. Bake for about 20 minutes or until the pastry is golden brown. Remove from the oven and let sit for about 5 minutes before slicing and serving.