Preheat the oven to 350° F. Lightly spray a 9" x 13" baking pan with nonstick spray and set aside. If desired, line pan with a parchment paper sling for clean cuts.
In a medium bowl, whisk together flour, powdered sugar, and salt. With a rigid pastry blender, cut in the butter until ½" sized pieces form.
Scatter the crust mixture evenly over the bottom of the prepared pan, then gently press into an even layer.
Bake the crust for 15 minutes, then remove from the oven.
While the crust is baking, stir together granulated sugar, flour, salt, and nutmeg in a medium bowl.
Add the eggs and use a spatula to mix until just evenly combined. Then fold in the rhubarb.
Pour the filling over the par-baked crust, spreading it evenly.
Bake at 350° F for 40 to 45 minutes or until the top is lightly browned and the center is not jiggly.
Transfer to a cooling rack to cool completely. Refrigerate until firm enough to cut, about 2 hours.
With an electric mixer, beat together the cream cheese, powdered sugar, sour cream, and vanilla extract until completely smooth.
Once chilled, cut into bars and serve topped with small dollops of the cream cheese mixture.
