To make the galette dough: pulse the flour, cornmeal, sugar and salt together in a food processor fitted with the blade attachment until combined. Add the butter and pulse until the mixture resembles coarse crumbs. Drizzle egg and pulse until a ball just begins to come together.
Form dough into a disk. Wrap in plastic and chill until firm, at least 2 hours.
To assemble the galette, roll the dough out on a well-floured surface. I like rolling it over my rolling pin and transferring it to a tart pan for very easy assemble, with the edges of the dough draping over the pan. This way I know I’m keeping enough dough to fold over after mounding the filling. If you don’t have a tart pan just transfer directly on a rimmed baking sheet and just be sure to keep a 1-2 inch border around the edge when mounding the fruit in the middle. Remember it doesn’t have to be even! Fruit juices likely to leak out so if you have parchment paper line the baking sheet with it.
Preheat oven to 400 F degrees. You can put rolled out galette dough back in the fridge, so it doesn’t get too warm and melty while you assemble the rest of the ingredients.
Slice your rhubarb stalks vertically in thinner strips (halves or thirds). Place them in a shallow baking dish, squeeze the lemon juice on them, sprinkle with a tablespoon of sugar and toss lightly to coat. Set aside to light macerate while you prep the rest of the ingredients.
Pit the dates and line the bottom of the galette dough with them, covering as much as the bottom as possible and cutting the date is sizes accordingly. It’s not perfection we’re going for just as much as the bottom covered as possible.
Slice the strawberries and cover the date layer with them. Sprinkle 1 tablespoon of sugar over the strawberries.
Melt the butter, transfer to a small and mix with the vanilla pasta.
Arrange rhubarb stalks on top of strawberry layer. If need be, trim them shorter so they will fit inside the galette. It’s a trial-and-error kinda situation.
Using a pastry brush, back of a spoon or anything that works for you, brush the rhubarb stalks with about half the butter-vanilla mixture (ok to eyeball it) over the rhubarb stalks and sprinkle with remaining tablespoon of sugar. Optional to add a vanilla beam sliced in half, to the topping of the galette, nestled in between the rhubarb stalks,
Fold the edges of the dough up and over filling, overlapping slightly. Brush dough with remaining half of vanilla-butter mixture and sprinkle dough with tablespoon of sugar.
Bake until galette filling is bubbling and crust is deeply browned, 40-50 minutes. Let cool slightly. Serve topped with vanilla ice cream.