Preheat the oven to 180c.
Make the flax egg by mixing the ground flax and water in a bowl - set aside to thicken.
Blitz the walnuts in a food processor to a chunky crumb.
Then add all remaining ingredients and blitz thoroughly to form a dough.
Line a large baking tray with baking paper.
Form the dough into a ball and roll into a circle approx
Transfer to the baking tray then spread the squash mix over the top - leave a 1 /1/2 cm around the edge with no filling.
Now add the caramelised onions, reserved roast squash, a handful of walnuts and dollops of the pesto (approx half).
Fold up the outer edges of the crust towards the centre of the galette.
Crimp them a little to seal.
Brush the crust with a little olive oil and then bake for 25 minutes.
Remove from the oven to cool a little.
Thin the remaining pesto out with a little water, then drizzle over the top.