Preheat the oven to 350 °F, and lightly grease an 8-inch round or square cake pan.
Measure the flour, baking powder, salt, cinnamon, and remaining half the orange zest in a mixing bowl. Stir together until well blended.
In a second bowl, beat the sugar and butter together until the texture is light and fluffy.
Add the egg as you continue to beat. The mixture should be smooth and lemon colored.
Stir in the sour cream, and then the orange juice.
Add half the dry ingredients, and stir to blend thoroughly. Repeat with the remaining dry ingredients. The batter will be fairly thick, but should be smooth in texture.
Transfer the batter to the prepared cake pan, and spread it evenly.
Stir the fruit and nut topping together one last time, and drop it onto the cake. You can choose to leave all of it on top, or use the back of a spoon to push some of it lower into the batter. As it rises, what you’ve pushed down will bake into the cake, while the rest will create a topping.
Bake for about 20 minutes in the oven. At that point, open the oven door and sprinkle the coarse sugar on top.
Continue to bake for another 20 minutes or so, until the batter in the middle of the cake is fully set.
Remove from the oven, and allow it to cool for about 10 to 15 minutes before serving.
It will keep well in the fridge, covered, for up to a week.