In a food processor, pulse flour, sugar, salt and butter. Add butter and pulse for about 1 minute, until the mixture forms pea size pieces. Slowly add ice water, 1 tablespoon at a time, and pulse until the mixture comes together. Transfer the dough onto a lightly floured surface and gently pat into a disk. Wrap in plastic and chill at least 1 hour.

Combine rhubarb with strawberries, sugar, lemon juice and zest, corn starch and vanilla seeds.

Roll out the dough on a lightly floured surface. Carefully transfer to a baking sheet lined with parchment paper.

Spread the fruit in the center of the dough leaving a 1 inch/4 cm border. Fold the edges of the dough up and over the fruit, overlapping slightly. Now put the whole baking sheet into the freezer, and chill the formed galette for about 15 minutes.

In the meantime preheat your oven to 375°F/190°C.

Remove the galette from the freezer, brush the dough with the beaten egg and sprinkle with coarse sugar. Bake until the crust is deep golden brown and the filling is bubbling, about 40-45 minutes. Serve warm with a scoop of vanilla ice cream.