Preheat the oven to 180ºC. Grease and line a 20cm cake tin.
First, prepare the base of the cake. Wash the oranges and slice them into 2mm discs with a very sharp knife (you can use a mandoline or a slicer if you have them). I leave the rind on, as when cooked like this it becomes edible, but if you prefer you can remove it.
In a small saucepan over a medium heat, melt the demerara sugar with two tablespoons water until it has dissolved. Simmer for a few minutes until the syrup begins to caramelise (you should smell and see the colour change to a light amber). Pour your syrup over the bottom of the cake tin. Arrange the slices of blood orange, as many as will fit in one layer in a pleasing pattern, on top of the syrup.
To make the batter, whisk the oil, sugar, salt, ricotta, citrus juice and zest together in a large mixing bowl. Add in the eggs one at a time and beat until smooth. Add in the dry ingredients and beat until smooth. Pour the batter into the prepared tin and bake for 40–50 minutes, until golden and just set.
Allow the cake to cool for five minutes, then run a knife around the edge of the tin and invert onto a wire rack or serving plate. Allow to cool completely before slicing.