To make galette dough, place flour and salt in a bowl. Using a pastry cutter or your fingers incorporate the butter into the flour until it resembles the consistency of small pebbles. Add the ice water and stir to combine into a shaggy ball. Pat into a flat disk. Wrap in plastic wrap and refrigerate for one hour.
Meanwhile, heat a wide bottom pan over medium heat. Add the onions, olive oil, thyme and salt. Cover and cook till onions are wilted and stewing in their own juices. Remove the lid and continue cooking over medium heat until all the liquid has cooked down and the onions are soft and light brown in color. Remove the thyme sprigs and allow to cool down. To prepare the squash, cut the rind off of the butternut squash and slice into thin slices. Heat a large non stick skillet over high heat. Add the olive oil, squash and season with salt and pepper. Cook, tossing often until squash is tender and starting to brown. Remove from the heat and allow to cool.
Preheat oven to 425ºF. To assemble the galette, roll out the dough on a well floured work surface into a round disk about a 1/8 inch thick. Place on a parchment lined sheet pan and chill for twenty minutes. Remove from the refrigerator and spread a layer of the onions in the center, leaving a two inch border. Cover with the butternut squash and top with the grated cheese. Fold the edges of the dough over to partially cover the filling. Place in the middle of the preheated oven and bake for 30-35 minutes until pastry is a golden brown. Remove from the oven and transfer to resting rack to cool. Cut into wedges and serve.
Enjoy with a glass of Cakebread Cellars Cuttings Wharf Chardonnay.