Step 1Heat the oven to 325 degrees. Grease a 13-by-9-inch baking pan.
Step 2Scrub the rind of the orange well. Remove both ends of the orange and cut it into large chunks. Set aside.
Step 3Place the raisins in a food processor and pulse 6 or 7 times until coarsely chopped. Add the orange chunks and pulse 5 or 6 times to reduce them to evenly sized small pieces. Empty into a small bowl and set aside.
Step 4Pulse the granulated sugar and shortening together in the food processor 4 or 5 times to combine, scraping down the sides of the bowl. Add the eggs, one at a time, and process in between, scraping down the sides. The mixture should be smooth, light and fluffy. Add three-fourths cup buttermilk and process to combine. The mixture will be very liquid and may look slightly separated.
Step 5Sift together the flour, baking powder, baking soda and cinnamon and add to the processor. Pulse just to combine into a smooth batter. Add all but 2 tablespoons of the orange-raisin mixture and pulse once or twice. Add the walnuts and pulse once; do not over-process.
Step 6Pour the batter into the pan; bake until the sides have begun to brown and pull away from the pan, 35 to 40 minutes. The cake should still be a little moist in the center.
Step 7Whisk together the remaining 2 tablespoons of buttermilk and the powdered sugar until smooth. Whisk in the reserved 2 tablespoons of the orange-raisin mixture and spread it over the hot cake. Serve at room temperature.