In a small bowl, combine the zest of a very large orange with the white sugar. Use a fork to work the zest into the sugar. This will release and infuse all the citrus oils into the sugar. The sugar will appear to have a light orange hue to it. That is what you need.
Preheat the oven to 325 °F. Line an 8X8 cake tin with parchment paper. Place the tin on a cookie sheet and set aside.
Add the infused sugar and the room temperature eggs into the bowl of a stand mixer with a wire whisk attachment. (If not, a large bowl with a hand mixer works fine here)Whip the eggs with the infused sugar on high for several minutes until the mixture grows more than triple more in size. The end result is a pillowy, thick, mousse-like texture.
With the mixer on low, slowly add the orange juice and oil and allow it to combine.
Remove the bowl from the mixer and use a silicone spatula and gently fold the almond flour into the batter until no flour remains visible (be careful not to deflate the eggs). Don't add the almond flour all at once. Spoon it in slowly.
Slowly pour the batter into the prepared pan and sprinkle the top with slivered almonds. Place the cake in the oven for roughly 35 minutes, until a toothpick comes out cleanly and the top of the cake is golden brown.
Remove the cake from the oven and allow it to cool in the tin on a wire wrack.
Once completely cooled, dust the top of the cake with powered sugar and enjoy!