Zest one orange, then cut all into 3mm thick slices. Remove the seeds.
Put the sugar and water in a large, shallow saucepan.
Stir over medium heat until the sugar has dissolved, then add the orange slices and gently simmer for 10-15 mins until the rinds are soft and beginning to turn translucent. Set aside.
Preheat the oven to 180°C and line the base & side of a 22-23cm springform cake tin with parchment paper.
Using tongs, arrange the orange slices in the base of the tin, overlapping each other slightly and going up to the side of the tin too (not fully, just a bit).
Brush the oranges with the orange syrup, using a silicon brush. Set the remaining syrup aside.
In a mixing bowl, combine the flour, ground almond, baking powder, orange zest, chopped rosemary leaves and salt.
In another bowl, beat the sugar and butter or dairy-free spread until pale and creamy. Add the eggs, one at a time, beating well between each addition. Stir in the almond milk.
Fold in the flour, mix until you get a smooth batter, but don’t overmix!
Tip the batter into the cake tin (over the orange slices).
Bake for 45 mins until the cake is golden and a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin for 5 mins, then loosen the parchment paper around the edge of the tin and carefully open the springform.
Transfer the cake to a wire rack and carefully peel off the paper from the side of the cake.
Turn over the cake and remove the paper completely.
Brush the cake with the remaining syrup.
Serve warm or let it cool completely, then serve.