Weigh out the butter and place it in the freezer 1 hour before you start making the pastry.

In a large mixing bowl, combine the flour, salt and sugar (if using).

Coarsely grate the frozen butter into the flour mixture and toss together to coat the butter pieces.

In a separate bowl, whisk the sourdough discard with 3 tablespoons of ice water (reserve the rest for later, should the dough be too dry).

Add the sourdough mixture to the flour and, using a blunt knife or fork, work it in to form a dough. If it's too dry, add a little more water, 1 teaspoon at a time, and knead just enough to bring it together.

Flatten the dough, wrap tightly in cling film and chill for at least 4 hours, or overnight.

Preheat the oven to 200°C (400°F).

Slice the squash into thin half-moons, about 5mm thick. Toss with the olive oil, salt and pepper, and spread out on a baking tray.

Roast for 18–20 minutes, until just tender and lightly coloured at the edges but not falling apart. Set aside to cool completely.

Melt the butter in a small frying pan over medium heat.

Add the sliced red onion and cook for 7–8 minutes, stirring occasionally, until soft and lightly sweetened. Season with a small pinch of salt. Set aside to cool.

In a bowl, combine the cream cheese, 125g of the soft goat's cheese, egg yolk, minced garlic, thyme leaves and salt. Mix until smooth.

Preheat the oven to 190°C (375°F).

Prepare the egg wash by beating the egg white with a drop of milk.

On a large sheet of parchment paper dusted with flour, roll the chilled pastry into a rough circle, about 35–40cm in diameter and 2–3mm thick.

Spread the cheese mixture over the pastry, leaving a 5cm border.

Arrange the roasted squash slices over the cheese, slightly overlapping.

Scatter the cooled red onion over and between the squash.

Tear the remaining 50g of goat's cheese into small pieces and dot over the top, tucking them among the squash and onion.

Fold the pastry edges over the filling, pleating loosely as you go, and brush the edges with egg wash.

Slide the parchment paper with the galette onto a baking sheet and bake for 40–45 minutes, until the crust is deep golden brown.

Leave the galette to cool and firm up for 10–15 minutes. Scatter over the walnuts and a few extra thyme leaves, and finish with a light drizzle of honey if using. Slice and serve warm or at room temperature.