Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.

In a mixing bowl, combine all-purpose flour, granulated sugar, and softened unsalted butter and mix until resembling coarse crumbs.

Press the mixture firmly into the bottom of the prepared baking dish.

Bake the crust for about 10-12 minutes until lightly golden, then let it cool slightly.

Beat cream cheese, granulated sugar, and vanilla extract until smooth, then spread it evenly over the cooled crust.

Toss diced rhubarb with sugar and flour, then layer evenly over the cream cheese mixture.

For topping, combine oats, brown sugar, and melted unsalted butter until clumpy, then sprinkle over rhubarb.

Bake for 35-40 minutes until topping is golden brown and filling is set.

Let cool completely before slicing into squares.