Preheat the oven to 350°F. Oil a 11 x 11-inch cake pan (or any medium square or rectangle baking dish).
In a large bowl mix together the oil and sugar. Beat the eggs into the bowl.
Squeeze the orange into the bowl and add the diced orange, continue to beat for a few seconds.
Add the ground almonds and hazelnuts, flour, baking powder, and chia seeds and mix with a wooden spoon until the batter is well combined and smooth.
Pour the batter into the pan, then gently place the orange slices over in a row with even gaps. Sprinkle some sugar and put into the oven for about 34 – 40 minutes or until a toothpick inserted in the center of the cake comes out clean. Let the cake cool completely on rack, then slice it into squares. Store the bars in a container for up to 3 days.