For the Crust
Fill a cup with ½ cup of water, and drop in a few ice cubes; set it aside. In a large bowl, whisk together flour, sugar and salt. Dice very cold unsalted butter into ½-inch pieces.
Sprinkle the butter cubes over the flour and begin working them in with a pastry blender or fork using it to scoop and redistribute the mixture as needed so all parts are worked evenly until all of the butter pieces are the size of tiny peas.
Start by drizzling ¼ cup of the ice-cold water (but not the cubes) over the butter and flour mixture. Using a rubber spatula, gather the dough together. You’ll probably need an additional ¼ cup of cold water to bring it together, but add it a tablespoon at a time. Once you’re pulling large clumps with the spatula, start using your hands to bring the dough together. Gather the clumps together into one mound, kneading them gently together. Form into a disk and wrap in plastic wrap. Refrigerate for at least 30 mins.
For the Filling
Heat a skillet over medium heat and add olive oil and butter. Add the squash, cinnamon and sugar, stirring to coat. Cook, stirring occasionally, until the squash is slightly tender. Remove the contents from the skillet and place them in a bowl with the fresh cranberries. Place the bowl in the fridge until ready to use.
Bake the galette until the crust is golden, about 45 to 50 minutes. Remove and let cool slightly before serving.