In a food processor, blend the quick oats until finely ground. Add the all-purpose flour, sugar, and kosher salt, pulsing to combine. Cut in the cold butter until the mixture resembles coarse crumbs, about pea-sized pieces. Gradually add ice water, mixing just until the dough starts to clump together.

Gather the dough into a ball and wrap it tightly in cling film. Chill in the refrigerator for 30 to 60 minutes.

Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

Once chilled, dust a clean surface with flour and roll out the dough into a 12-inch circle, keeping it about 1/8-inch thick. Carefully transfer onto the prepared baking sheet.

In a large mixing bowl, toss the diced rhubarb with sugar, cornstarch, and lemon juice until evenly coated.

Spoon the rhubarb mixture into the center of the rolled dough, leaving a 1.5-inch border. Fold the crust edges over the filling, creating a rustic look. Brush with beaten egg and sprinkle with turbinado sugar.

Bake for 40 to 50 minutes, or until the crust is golden and the filling is bubbling.

While the tart cools for 20 to 30 minutes, beat the cream until soft peaks form. Add sugar and whip until stiff peaks are achieved.