Combine all of the pastry ingredients except the water in your food processor. Mix until the consistency resembles bread crumbs, then add the water a little at a time until the dough just comes together in a rough ball.
Remove from the processor, wrap in cling film and chill in the fridge for at least 10 minutes.
Meanwhile, cook onions, sugar and salt in the olive oil and butter, on med-high heat until lightly golden brown, about 15 minutes. Remove and allow to cool.
At this point, preheat oven to 170°C. Roll out your pastry dough between two sheets of baking paper, until about 1/2 cm thick.
The shape should be as close to round as possible, and you can trim it with a knife to help with the shaping. Lift off the top sheet of baking paper and spoon the onion filling into the centre of the pastry circle, leaving a border of at least 3cm.
Scatter over pieces of the Gorgonzola and the rosemary sprigs and fold up the sides of the pastry, gently pressing together where there are any overlaps.
Slide the baking paper with the galette on it onto a baking tray and place in the freezer for about 10 minutes. Bake at 170°C for about 20-25 minutes until the pastry is golden and cooked through.
Reprinted with permission of Sarah Graham
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