Mix flour with granulated sugar in stand mixer. Add butter and combine until crumbs form. With mixer running, slowly add ice water and lemon juice. Continue mixing until just until dough comes together.
Preheat oven to 375°F. Lay a large piece of parchment paper on counter and sprinkle with flour. Pat pastry dough into a ball. Dust rolling pin with flour and roll pastry on parchment into a circle, about 12 in. wide. Pastry will be thin and edges uneven. Transfer parchment and pastry to baking sheet.
Combine granulated sugar, cornstarch and salt in a large bowl. Stir in peeled and sliced apples and chopped rhubarb until coated. Tumble mixture onto centre of pastry. Fold pastry over, just to cover edge of fruit. Centre of galette should not be covered with pastry and edges will be uneven. Lightly brush pastry with water and sprinkle with course sugar.
Cover the exposed fruit with foil and bake until pastry is just about golden and mixture is starting to bubble, 25 min. Remove foil and bake 15 more minutes.
Serve warm with vanilla ice cream.