Preheat oven to 400°F. Roll dough into a rough 12-inch circle on parchment.
Toss strawberries, rhubarb, sugar, cornstarch, vanilla, cinnamon, and lemon in a bowl.
Pile fruit in center of dough, leaving a 2-inch border. Fold edges over fruit, pleating as needed.
Make crumble: cut cold butter into flour and brown sugar until crumbly. Sprinkle over fruit.
Brush crust with beaten egg and sprinkle coarse sugar if you want crunch.
Bake 35–40 minutes until crust is golden and juices bubble. Cool 15 minutes before slicing.