Preheat the oven to 180°C conventional or 160°C fan. Line and grease a 22 cm round cake tin.
Core the topping apples and cut them into 1–2 mm slices. Toss with lemon juice. Microwave for 45–60 seconds if the slices are too firm to bend.
Peel, core, and grate the small apple. Gently remove only the excess juice.
Whisk the eggs and sugar for two minutes, until smooth and lightly foamy.
Whisk in the cooled melted butter, milk, yogurt, and vanilla.
Mix the flour, baking powder, and salt in a separate bowl. Fold them into the wet ingredients just until combined.
Fold in the grated apple. Pour the batter into the prepared tin and smooth the surface.
Arrange the apple slices in overlapping circles, beginning at the outside edge and working toward the centre.
Mix the sugar and cinnamon and sprinkle over the apples. Brush lightly with melted butter if desired.
Bake for 30–35 minutes, until the centre is set and a skewer inserted into the batter comes out clean or with a few moist crumbs.
Cool in the tin for 15 minutes, then remove and cool for at least another 15 minutes before slicing.
For the optional glaze, mix the apricot jam with hot water and brush a thin layer over the warm apples.