Sprinkle a large sheet of parchment paper lightly with flour. Roll out the dough on the parchment, lifting the dough and lightly flouring the parchment as needed to prevent sticking. You're aiming for a 11- to 12-inch round (it should be about ⅛-inch thick). If the dough cracks, just patch it up as needed. Brush off any excess flour from the parchment paper, then slide the sheet of parchment (with the dough) onto a baking sheet.
Spread the ricotta mixture evenly over the dough, leaving a 1 ½-inch border. Arrange the kale over the ricotta, followed by the butternut squash and goat cheese. Fold the dough up over the filling, overlapping it and pinching it together (patch up any tears as needed). Refrigerate the galette for 5-10 minutes.
Beat the egg in a small bowl with the water. Brush the crust lightly with the egg wash (you won’t need it all). Bake the galette, rotating once, until the crust is a deep golden brown, about 30-32 minutes.
Transfer the pan to a rack and let the galette cool (you can serve it warm or at room temperature). If you’d like, drizzle the inside of the galette with a bit of truffle oil (swoon!). Before serving, garnish the galette with fried sage leaves, if using.