Slice the oranges, taking care to make each one about 1/2cm thick. Tip 200g of the caster sugar into a saucepan, along with 150ml water. Bring the pan to a simmer, then gently cook the orange slices for 10 minutes. Remove the fruit from the sugar syrup and leave to cool.
Heat oven to 180C/160C fan/gas 4. Whisk the butter and remaining sugar together until light and fluffy, then gradually mix in the eggs, one at a time. Add the orange zest, juice and vanilla extract, then beat in the flour and almonds. The batter needs to have a thick consistency that drops off a wooden spoon – add a splash of milk to loosen if necessary.
Grease and line a round, 23cm cake tin, then arrange the orange slices across the bottom of the tin. Spoon over the cake mix, level with the back of a spoon and bake for 35-40 minutes, or until a skewer inserted into the centre of the cake comes out clean.
While the cake cooks, heat the caster sugar, honey, orange juice and rosemary in a small saucepan until the sugar has melted and the liquid is just-starting to bubble. Set the syrup aside to infuse for 10 minutes.
Turn the cake out onto a plate or board while it is still warm, then drizzle the syrup over the cake.