Preheat the oven and mix the flour, oats, and brown sugar in a bowl, then cut in the cold butter until the mixture becomes crumbly.
Reserve part of the crumb mixture for the topping and firmly press the remaining mixture into the bottom of a greased baking pan.
Beat the cream cheese, sugar, salt, cinnamon, and nutmeg together until smooth and creamy without overmixing.
Add the egg and vanilla extract, mixing on low speed just until fully combined and smooth.
Fold the diced rhubarb gently into the filling so it stays evenly distributed throughout the mixture.
Spread the filling evenly over the prepared crust, making sure it reaches all corners of the pan.
Sprinkle the reserved crumb topping evenly across the surface for a golden, buttery finish after baking.
Bake until the topping is golden brown and the filling looks set with a slight wobble in the center.
Cool the bars completely on a wire rack before refrigerating until fully chilled and firm enough to slice neatly.