Prepare the pastry dough by combining the flours, 1 tbsp sugar, and salt in a food processor and mix until blended. Blend in butter pieces until it looks like corse sand. In a small bowl whisk 1 egg and milk together, then pour into the food processor all at once and pulse until a rough dough forms (do not over mix). This is a fairly moist dough, see notes in blog post above.

Lightly flour a work surface and form dough into a ball. If making 1 large galette press the dough into a disc, wrap in plastic wrap and refrigerate for at least 1.5 hrs, or up to 2 days. If you're making 4 mini galettes, break the ball of dough into 4 equal pieces (I used a scale, but eyeballing it would be fine too). Flatten each of the 4 pieces of dough into discs, wrap in plastic wrap and refrigerate for at least 1 hour, up to 2 days.

Once dough is chilled preheat oven to 375°F and line a large baking tray with parchment paper.

Remove dough from fridge and let sit on the counter for 10 minutes.

Meanwhile toss berries, cornstarch, 1/2 cup sugar, lemon zest, lemon juice and salt together in a large bowl. Set aside.

On a lightly floured surface roll out each disc of dough into 1/8" thick (or roll out into one large piece 1/8" thick). Transfer dough to baking sheet and top with blueberries, using around 3/4 cup per mini galette. Fold the outer edges of the galettes up creating small overlaps as you go. Brush the edges with milk and sprinkle on demerara sugar.

Bake for 40-50 minutes, until galettes are golden brown and the filing is bubbling (if you're making a larger galette the timing will be 50-60 minutes). Let galettes cool on baking sheet.

To make the mascarpone whip, combine mascarpone, whipping cream, and 3 tbsp sugar in a bowl and whisk until stiff peaks form (this took about 4 minutes with an electric stand mixer).