Preheat the oven to 180C and grease and line a 20cm round cake tin.
Cut the top and bottom the off the oranges, then standing on one of the flat surfaces slice down the sides to remove the rind and pith following the curve of the orange. Now turn on its side and slice the oranges into circles around 1/2 cm thick.
In a small saucepan add the caster sugar and water. Over a medium heat, use a wooden spoon to gently encourage the sugar dissolve, then bring to a simmer without touching the liquid until it has turned a light caramel colour. Pour the caramel glaze into the cake tin and arrange the orange slices on top, being careful of the hot caramel. Set aside.
In a food processor, blitz the pistachios until finely chopped, careful not to let them become greasy. Set aside.
Using an electric mixer, beat the butter, sugar and orange zest until light and fluffy. Add the eggs one after the other, beating well in-between eggs, until fully incorporated. In another bowl, whisk together the salt, flour, pistachios and baking powder, then gently fold into butter mixture. Spread and smooth this over the oranges in the cake tin.
Bake for 30 – 40 minutes or until an inserted skewer comes out clean. Allow to sit for 10 minutes in the tin, then carefully invert the cake onto a wire rack until cool. Serve with crème fraiche and extra pistachios.