PASTRY: Whirl flour and salt in a food processor until combined. Add butter and pulse until coarse crumbs form. While pulsing, gradually add sour cream and egg yolk through the feed tube. Pulse until dough comes together. Wrap in plastic wrap then flatten into a disc. Refrigerate until firm, about 1 hour.

PREHEAT oven to 375F and line a baking sheet with parchment.

ROLL out dough on a lightly floured surface to a scant ¼-in. thickness. Use a 4 ½ -in round cookie cutter to cut dough into 10 rounds. Re-roll scraps of dough at least once, refrigerating for 15 min if too soft to roll. Refrigerate rounds on prepared sheet until firm, about 15 min.

FILLING: Toss rhubarb, sugar, flour and salt in a medium bowl until combined. Scoop 2 heaping tbsp of rhubarb mixture onto centre of each round. Whisk beaten egg and 1 tsp water in a bowl and brush edges of rounds. Put remaining egg wash in fridge. Fold dough over the filling to form a half-circle, pressing to seal. Crimp edges with a fork, then return to baking sheet and refrigerate 20 min.

CUT small slashes in top of each pie. Brush with remaining egg wash and sprinkle with coarse sugar. Bake until puffed up and golden brown, about 20 min. Transfer to a wire rack to cool for 15 min. Pies will keep well at room temperature in a covered container up to 2 days. Reheat in a 250F oven for 10 min, if desired.