Cook the polenta according to the instructions on the packet. Stir in the caster sugar and orange zest, then pour into a lined baking tray and spread out with the back of a spoon. Allow to cool. Cover, then chill in the fridge to set for at least 4 hours, or overnight if you can.
Once the polenta is set, cut each apple into 8 wedges and remove the cores. Transfer to a small pan with the apple juice, cinnamon, mixed spice and brown sugar. Bring to the boil and simmer for 4-5 minutes, until the apples are slightly tender but still holding their shape. Remove from the pan with a slotted spoon, then bubble the sauce over a medium heat for 10-12 minutes, until thickened and syrupy.
Meanwhile, cut the cold polenta into 6 squares. Heat a large non-stick pan with the oil. Add the slices of polenta in batches and cook for 2-3 minutes on each side, until golden brown, transferring to a low oven as you go.
Serve the polenta topped with the yogurt, spiced apples and syrup.