In a food processor, blend quick oats until they resemble flour. Add all-purpose flour, sugar, kosher salt, and cold butter, pulsing until the mixture forms pea-sized crumbs. Gradually add ice water, one tablespoon at a time, until the dough just clumps together. Wrap the dough in plastic wrap and chill in the refrigerator for 30 to 60 minutes to ensure a flaky texture.

Preheat your oven to 400°F (200°C). On a floured surface, roll out the chilled dough into a 12-inch circle, about ¼-inch thick. Carefully transfer the rolled dough to a parchment-lined baking sheet, making sure to mend any cracks that may form during the process.

In a mixing bowl, combine the diced rhubarb with sugar, cornstarch, and lemon juice, tossing until thoroughly coated. Spoon the rhubarb filling into the center of the dough circle, leaving a 1½-inch border clear for folding.

Gently fold the edges of the crust over the rhubarb filling, overlapping as necessary to create a rustic edge. Brush the exposed crust with a beaten egg and sprinkle turbinado sugar over it.

Place the baking sheet in the preheated oven and bake for 40 to 50 minutes.

Once baked, remove the tart from the oven and let it cool for 20 to 30 minutes on a wire rack. Slice and serve warm or at room temperature.