To make the yoghurt, mix together all the ingredients in a small bowl. Cover and refrigerate for at least 2 hours, preferably overnight.
Preheat the oven to 220C/200C Fan/Gas 7 and line a baking tray with baking paper.
Cut the puff pastry into quarters (they should be about 10x13cm/4x5in) and place on the lined baking tray.
Divide the jam between the pastry and spread with the back of a spoon, leaving a 1cm/½in margin all the way around each piece. Lay the apple slices, overlapping, on top of the jam. Brush the exposed edges of the pastry with the milk.
Mix together the sweetener, cinnamon and mace and sprinkle liberally over each tart.
Bake for 15–20 minutes, or until the pastry is golden-brown and crispy.
Serve with a generous spoonful of the yoghurt on the side.