Preheat oven to 350°.
To prepare crust, lightly spoon 1 1/2 cups flour into dry measuring cups; level with a knife. Combine 1 1/2 cups flour, 1/2 cup sugar, and salt in a bowl. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Press mixture into a 13 x 9-inch baking dish coated with cooking spray. Bake at 350° for 15 minutes or until crust is golden brown.
To prepare filling, lightly spoon 1/3 cup flour into a dry measuring cup; level with a knife. Combine 1/3 cup flour and 1 1/2 cups sugar in a large bowl; add milk and eggs, stirring with a whisk until well blended. Stir in rhubarb. Pour rhubarb mixture over crust. Bake at 350° for 40 minutes or until set. Cool to room temperature.
To prepare topping, place 1/2 cup sugar, cheeses, and vanilla in a bowl; beat with a mixer at medium speed until smooth. Gently fold in whipped topping; spread evenly over baked custard. Cover and chill at least 1 hour. Garnish with mint sprigs, if desired.