Preheat the oven to 180C/350F and prepare round cake tin by greasing it and lining the bottom with some parchment paper.
Peel and thinly slice two or three oranges (depending on their sizes).
Sprinkle demerara sugar evenly on the bottom of the cake tin and arrange orange slices on top (they can overlap slightly). Make sure you put orange slices "neat cut" down as they will be visible once the cake is turned upside down.
Put all the dry ingredients in a large bowl and give them a whisk. Alternatively, sift them through a sieve.
Zest one orange and add it too. Whisk it in so that it's combined with all the dry ingredients.
In a large jug mix together oil, yogurt and orange juice until combined.
Add wet ingredients into the bowl with dry ones and mix gently until just combined (try not to overmix it). Your cake batter will be pretty stiff and lumpy which is as it should be.
Pour cake batter on top of your orange slices (make sure you do it gently not to mess up your neatly arranged fruit). Spread it out nice and even.
Bake your cake in preheated oven for 35-40 minutes until nice and golden and skewer inserted into centre comes out clean.
Take the cake out of the oven and let it stand for 10 minutes before turning it out upside down onto a plate or cooling rack and gently peeling off the layer of baking paper.
Let it cool and serve!