Preheat oven to 350 degrees F (175 degrees C).

Combine flour and 1/2 cup sugar in a 9x13-inch baking pan; cut in butter until mixture is crumbly. Press firmly into an even layer.

Bake in preheated oven until crust is slightly browned, about 25 minutes.

Place 1/3 of the nectarine slices in a blender and puree until smooth. Stir cornstarch into the mixture until evenly distributed.

Beat eggs in a large bowl until light, using an electric mixer; gradually beat in 1 cup sugar until well combined. Stir in the blended nectarine mixture; pour over hot baked crust.

Bake until almost set, about 30 minutes. Let cool completely on a rack, 30 minutes to 1 hour; cut into bars. Top bars with remaining nectarine slices.