Prepare the Dough: In a food processor, pulse the flour and salt. Add the cold, cubed butter and pulse a few times until the mixture resembles coarse crumbs. Drizzle in the ice water and pulse until the dough just comes together. Do not overmix. Form the dough into a disc, wrap in plastic, and chill for at least 30 minutes.

Make the Filling: In a large bowl, combine the chopped rhubarb, strawberries, sugar, cornstarch, vanilla extract, and lemon juice. Mix well until the fruit is evenly coated.

Assemble the Galette: On a lightly floured surface, roll out the chilled dough into a large, rough circle, about 12-14 inches in diameter. Carefully transfer the dough to a baking sheet lined with parchment paper.

Add the Filling: Spoon the strawberry rhubarb filling onto the center of the dough, leaving a 2-inch border around the edges.

Fold the Edges: Carefully fold the empty dough border up and over the outer edge of the filling, pleating as you go. This will create a rustic, free-form crust.

Bake the Galette: In a small bowl, whisk the egg and milk to create an egg wash. Brush the egg wash over the folded crust and sprinkle the coarse sugar over the top. Bake at 400°F (200°C) for 35-40 minutes, or until the crust is golden brown and the filling is bubbly and thick.

Serve: Let the strawberry rhubarb galette cool on the baking sheet for at least 15 minutes before slicing. Serve warm, on its own or with a scoop of vanilla ice cream.