Prepare the oven and pan. Preheat the oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper.
Prepare the apples. Mix the grated apples with the cinnamon and 3 tablespoons taken from the measured ¾ cup (150 g) sugar. Spread evenly over the parchment-lined pan.
Beat the eggs and sugar. In a mixing bowl, beat the eggs with the remaining sugar until pale and fluffy.
Finish the batter. Gently mix in the vanilla and milk. Combine the flour and baking powder, then gently incorporate them into the egg mixture just until combined.
Cover the apples. Pour the batter over the apple layer. Carefully spread it to cover all the apples evenly.
Bake. Bake for approximately 20 minutes, until golden and a toothpick inserted into the sponge layer comes out clean.
Rest and flip. Remove from the oven and let rest for 5 minutes. Trim uneven edges if desired. Cover with a fresh sheet of parchment and a second baking sheet or flat tray. Using oven mitts, carefully flip the cake. Remove the original pan and peel off the parchment covering the apples. Remove any trimmed borders.
Roll while warm. Use the fresh parchment underneath to gently lift and roll the cake into a log, with the apples inside. Keep the paper outside the spiral.
Cool and finish. Leave seam-side down to cool completely. Remove the parchment, dust lightly with powdered sugar, and decorate with walnuts if desired. Slice with a sharp knife.