In the bowl of a food processor add the oats. Blend until finely ground. Add the flour, sugar, and kosher salt. Pulse a few times to mix the dry ingredients. Cut the cold butter into small pieces and add to the food processor. Pulse until the mixture resembles small pea-sized pieces. Pour the ice water through the chute and pulse just until the dough begins to clump together.
Dump the contents of the food processor onto the counter and form into a ball. Wrap with cling film and let chill for 30 to 60 minutes.
Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper and set aside. On a lightly floured surface roll the tart dough out into a 12-inch circle. Be gentle while rolling the dough to avoid making large rips – because this tart is free-formed, any cracks will cause the filling to leak.
Gently transfer the dough to a parchment-lined baking sheet. Carefully mend any cracks in the dough as best you can by pinching the dough together. Toss the rhubarb with the sugar, cornstarch, and lemon juice. Keep mixing the rhubarb until the dry ingredients are moist and cling to the fruit and there isn't any more dry powder. Spoon the rhubarb filling into the middle of the dough. Spread the filling out evenly while leaving a 1 ½ inch edge border.
To form the tart, begin lifting crust up over the edge of the berries, overlapping and pleating the dough as you go. If the dough tears, then pinch it back together as best you can. Brush the edge of the crust with the beaten egg and sprinkle with turbinado sugar.
Bake the tart for 40 to 50 minutes until the pastry is golden browned and the rhubarb filling is bubbling and slightly thickened. Let the tart cool for 20 to 30 minutes before slicing.
In the bowl of a mixer fitted with a whip attachment, add the whipping cream. Begin to whip the cream on high until it begins to thicken into soft peaks. Add the sugar and continue whipping until the cream has thickened into stiff peaks. Dollop the cream on slices of the rhubarb tart and enjoy!