Begin by blending the quick oats in a food processor until they reach a fine consistency. In a mixing bowl, combine these oats with all-purpose flour, sugar, and salt. Cut in the cold butter until the mixture resembles pea-sized pieces, then gradually add ice water until the dough clumps together. Form it into a ball, wrap in plastic, and chill in the refrigerator for 30-60 minutes to firm up.

Preheat your oven to 400°F (200°C) as you roll out the chilled dough. On a lightly floured surface, use a rolling pin to stretch the dough into a 12-inch circle. Carefully transfer the dough onto a baking sheet, mending any cracks with your fingers.

In a large bowl, toss the diced rhubarb with sugar, cornstarch, and lemon juice until all pieces are well-coated. Spoon the filling into the center of the rolled-out dough, leaving a bit of the edge uncovered.

Gently fold the edges of the crust over the filling, creating pleats as needed. Brush the exposed crust with the beaten egg. Sprinkle turbinado sugar over the crust.

Place the assembled tart in your preheated oven and bake for 40-50 minutes until the crust is golden brown and the filling is bubbling. Adjust the position in the oven if necessary.

Allow the tart to cool on a wire rack for 20-30 minutes before slicing. Whip the cream until soft peaks form, then add sugar until stiff peaks. Serve over each slice.