(Optional step) Roast the walnuts for 10 minutes in the oven, turning halfway through.
Grind the nuts to a fine powder (leave them to cool down if roasted). Then add the flour and half the cinnamon and grind some more.
Add the butter and mix until you get a breadcrumbs like consistency (you can put the butter for 10 minutes in the freezer beforehand if you like to make sure it is cold enough).
Transfer everything to a bowl and add 3 tablespoons of very cold water. This is where you need to judge if the dough needs more flour if it’s too wet. Once it all comes together nicely, rest the dough for at least 1 hour in the fridge.
After the hour is up, roll the dough nicely, not too thin. You may need more flour, as the butter starts to melt. Transfer it to a baking tray lined with baking paper (you can roll the dough directly on the tray to save some time).
Sprinkle the lemon juice over the blueberries, then lay them on top of the dough, right in the centre, leaving about 4 cm from the margin.
Start folding the galette dough from the margin inwards. Sprinkle the rest of the cinnamon over the edges of the galette.
Bake for 40 minutes at 180 C (fan)/ 200 C (electric)/ Gas mark 6/ 400 F, until nicely browned.