1 tsp softened butter, for greasing
420 gm castor sugar
375 gm blanched almonds or 375 gm almond meal
1 heaped tsp of baking powder
Icing sugar and double cream, to serve
Combine 3 of the oranges in a large saucepan with enough cold water to cover generously, bring to the boil over medium-high heat, reduce heat to medium and simmer, shaking the pan occasionally, until very tender (2 hours). Drain, then set aside for the oranges to cool.
Meanwhile, preheat the oven to 160°C and prepare 24cm-diameter spring-form cake tin by greasing with the butter and lining the bottom with a round of baking paper. Trim the ends the remaining oranges (discard), then very thinly slice and remove any seeds. Arrange the orange slices in the base of the prepared cake tin in concentric circles, overlapping the slices slightly as you so, set aside.
If using whole almonds, blitz them in a food processor to form a fine almond meal and set aside.
Halve the cooked oranges, remove any seeds, then coarsely chop the oranges and process in a food processor to a smooth puree.
Whisk the eggs and sugar in a bowl to just combine, then stand until sugar dissolves (4-5 minutes). Fold in the orange puree, then the almond meal and baking powder and pour the batter into the prepared cake tin. Bake until golden and the centre of the cake is firm when pressed gently with your fingertip (50-60 minutes), cool in the cake tin for 10 minutes then turn out onto a wire rack. Burnish the orange slices on the cake with a kitchen gas torch (optional), dust with icing sugar and serve with dollops of double cream.