If using frozen berries, start by thawing them in a mesh strainer set over a bowl while you prepare the dough. Zero waste tip: use any juices you catch in the bowl for a smoothie!
Prepare the dough: place the flours, cane sugar, salt and buttery sticks in the food processor and pulse until the butter is in pea-like pieces and the mixture looks like sand, about 10-12 pulses. Then, drizzle the water into the food processor while pulsing, just until dough comes together - it’s quick! Look too dry? Drizzle in more water, 1 tablespoon at a time.
Flour your surface, and knead the dough a few times until it comes together in a flattened disk. If dough feels a bit too wet or sticky, just knead a little more flour until it's no longer sticky. Wrap disk in plastic wrap and chill in the fridge for one hour.
Preheat the oven to 400℉ (205℃). Cut a large piece of parchment and lightly flour it. Place chilled dough on floured parchment and roll out to roughly 10 x 14 inch square. No need to get it perfect, rustic is what we’re after. Transfer the parchment to a rimmed baking sheet.
Make the filling: how much sugar you add to the filling is up to you. Taste your blackberries first, and if they’re super sweet, just add 1-2 tablespoons for flavour enhancement. If they’re tart, use a full ¼ - 1/3 cup of cane sugar or your favourite granulated sugar replacement.
In a medium bowl, gently toss berries with sugar, cornstarch, lemon zest and salt. Pour into the centre of the dough, spreading it out leaving about 2 inches (5cm) border. Then, fold that dough over the filling, so it forms a shallow "bowl" over the filling, pressing gently where the dough folds over itself to help seal it.
Bake for 45-55 minutes, until filling looks jammy and bubbly and dough is golden brown and crisp. Check after 45 minutes, and every few minutes after that. Let cool for at least 10 minutes before slicing and serving.
Galette will keep on the counter for a day or in the fridge, wrapped, for up to 3 days.