Preheat the oven to 350°F.

In a mixing bowl, toss rhubarb with granulated sugar, cornstarch, and vanilla.

Pour the rhubarb mixture into a greased 8x8 baking dish.

In another bowl, mix oats, flour, brown sugar, melted butter, and salt until crumbly.

Sprinkle the oat mixture evenly over the rhubarb.

Bake for 35–40 minutes, or until the top is golden and the filling is bubbling at the edges.

Let it cool for a bit before serving (it’s hot lava straight out of the oven).