1) Pulse the flour and salt together in a food processor. Add the butter and pulse about 10 times until the mixture resembles coarse polenta with a few bean-size bits of butter in it.
2) Add the egg and pulse 1 to 2 times more; don't let the dough form a mass around the blade. If the dough seems very dry, add up to 1 tbsp of cold water, 1 tsp at a time, pulsing briefly.
3) Remove the blade and bring the dough together by hand. Shape the dough into a disk, wrap it in cling film, and refrigerate at least 1 hour.
For the filling:
1) Halve and core the apple. Cut each half into 8 wedges and put them in a large bowl. Slice the squash and cut the onion into wedges so that both are as thick as the apple wedges and add them to the apples.
2) Add the butter, rosemary and thyme and toss gently to combine. Season with salt and pepper and toss again.
To assemble and bake the galette:
1) Preheat the oven to 200C/Gas Mark 6.
2) Roll the dough on a lightly floured surface into a 30-cm disk. Transfer the dough to a baking sheet and brush with mustard. Starting 5-cm from the edge, casually alternate pieces of apple, squash, and onion in overlapping circles – if you have extra pieces of one or another, tuck them in where you can or double them up to use all the filling.
3) Fold and pleat the dough over the edge of the filling. Bake until the crust is brown and the apple, squash, and onions are tender and caramelised, about 55 minutes. Alternatively, transfer to a freezer-safe container to cook from frozen later. Please note, cooking times will increase when cooking from frozen and you may need to lower your oven temperature slightly.
4) Scatter the cheese over the filling and bake until melted, about 5 minutes more. Cool the galette briefly on a wire rack. Cut into wedges and serve.
Cook's Note: Don't be afraid to cook this galette – or any of your pies or tarts, for that matter – until the crust is a rich golden brown. A pastry's buttery taste and flaky crispness really come through when it is fully cooked.