In a food processor, grind the quick oats until they reach a fine consistency. Combine them with all-purpose flour, sugar, and kosher salt in the processor, pulsing until well mixed.

Add cold, cubed butter to the dry mixture and pulse until it resembles small pea-sized pieces.

Gradually add ice water, one tablespoon at a time, pulsing until the mixture starts to clump together. Gather and form the dough into a ball, then wrap it tightly and chill for 30 to 60 minutes.

Preheat your oven to 400°F (200°C).

Once chilled, turn the dough out onto a lightly floured surface and roll it out into a 12-inch circle.

Carefully transfer the rolled-out dough to a baking sheet lined with parchment paper.

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

Spoon the rhubarb filling into the center of the dough, leaving a 1 ½-inch border around the edges.

Gently fold and pleat the edges of the dough over the filling.

Brush the exposed crust with a beaten egg and sprinkle turbinado sugar over the top.

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

Once baked, allow the tart to cool for 20 to 30 minutes before slicing.

While the tart cools, whip cream until soft peaks form. Add sugar to taste.