Put the flour, salt and butter in a food processor and pulse until it forms a breadcrumb-like texture. Add the mascarpone, lemon juice, zest and water and pulse until it comes together as a ball. You can also make by hand by rubbing the butter into the flour then mixing in other ingredients.
Remove the dough from the food processor, bring it together in a ball then wrap in plastic wrap/cling film. Chill ideally for at least 30 minutes. Note you can make it a day or two ahead.
To make the galette
Preheat oven to 400F/200C.
Toss the blueberries with the lemon zest and juice, cornstarch and sugar.
Roll out the pastry into roughly a circle approx 12in/30cm diameter. Transfer the pastry to a baking sheet/tray. Put the blueberries in the middle and spread out into an even layer leaving around 1 ½ in/ 6cm around the edge.
Fold in the pastry edge, making pleats as you go round at even intervals, so the pastry partially covers the blueberries. Seal up any cracks that may form.
Brush the pastry that you can see from the top with egg then sprinkle over the sugar.
Bake the galette for around 25 minutes until the pastry is crisp and golden (bake a little longer if it isn't). Serve warm or room temp.