Preheat your oven to 350°F (175°C).

Combine flour, oats, sugar, and salt in a large mixing bowl. Cut in butter until the mixture resembles gravel-like crumbs.

Press half of the crust mixture into the bottom of a greased baking pan and bake for about 10 minutes until lightly golden.

Blend cream cheese, sugar, and egg until smooth. Add vanilla, cinnamon, and nutmeg, then blend until combined.

Fold in diced rhubarb and spread over the pre-baked crust.

Sprinkle the remaining crust mixture over the cream cheese filling.

Bake for approximately 35-40 minutes, until the topping is golden and the filling is set but slightly jiggly in the center.

Allow to cool in the pan on a wire rack for at least 30 minutes before slicing into squares.