1. Preheat the oven to 350 degrees. Grease a 9"x13" pan.
2. In a stand mixer, whip together the soft butter and 1/2 cup of sugar. When the mixture has been creamed to a pale yellow color, add 2 cups of flour and a hearty pinch of salt to the bowl; blend together on low speed to a crumbly texture.
3. Press the crust into your pan, pinching some of the dough along the walls of the pan. Place the dough in the oven and bake for 15 minutes; remove from the oven and let cool while you prepare the filling.
4. Zest your oranges, squeeze the fresh juice from the limes and oranges, and whisk together the remaining cup of flour, 3 cups of sugar, eggs, and fresh juice and zest. Pour the filling over the crust, return to the oven, and bake for 20 minutes.
5. While it is baking, cut off the outer layer (pith and all) of the last 2 oranges, and slice into thin rounds. After 20 minutes of baking, remove the pan from the oven and top with the sliced oranges in a single layer.
6.. Place back into the oven and bake for another 40-55 minutes, or until the center is set. Remove from the oven, and let cool completely before serving.