Preheat your oven to 350°F (175°C). Grease a 9-inch tart pan lightly with butter or non-stick spray.

In a mixing bowl, combine fresh, chopped rhubarb with sugar and let sit for 15–20 minutes. Drain excess liquid, then stir in vanilla paste and cornstarch.

Cream together softened butter and sugar until light and fluffy, about 3–5 minutes. Beat in vanilla and salt, then gradually add flour until a soft dough forms.

Press about two-thirds of the shortbread dough into the bottom and up the sides of the prepared tart pan. Chill in the refrigerator for 10 minutes.

With the remaining dough, add rolled oats and mix until it creates coarse crumbles.

Pour prepared rhubarb filling over the chilled crust and sprinkle oat mixture on top.

Bake in the preheated oven for approximately 35 minutes until the crust is golden brown and the filling is bubbling. Cover loosely with foil if browning too quickly.

Allow the tart to cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.