Preheat your oven to 350°F (175°C). Grease a 9-inch square baking pan liberally.

Combine all-purpose flour, old-fashioned oats, and brown sugar. Cut in cold butter until the mixture resembles coarse crumbs. Reserve 1 cup of this mixture for the topping.

Press the remaining mixture into the bottom of your greased baking pan. Bake for 10 minutes until light golden brown.

In a large mixing bowl, beat together cream cheese, granulated sugar, salt, ground cinnamon, and ground nutmeg until smooth. Add in the beaten egg and vanilla extract, mixing until just combined.

Gently fold the diced rhubarb into the cream cheese mixture and ensure it’s evenly distributed.

Pour the creamy rhubarb filling over the pre-baked crust, ensuring it reaches the edges of the pan.

Sprinkle the reserved crumb mixture over the top of the filling.

Bake for 40 minutes until the topping is golden brown and the filling is set but slightly wobbly.

Cool on a wire rack for one hour and then refrigerate for at least two hours before cutting into squares.