Preheat the oven to 170°C (150° fan) | 325F | gas 3. Grease and line a 23 cm | 9" cake tin with greaseproof paper.
Arrange the orange slices in the base of the tin, overlapping them in two layers so that it covers the base entirely.
In a large oil-free mixing bowl, beat the egg whites with a pinch of salt until stiff. Gradually whisk in the sugar until incorporated and then continue beating until the mixture is thick and glossy.
In a separate mixing bowl, beat the egg yolks with the orange blossom water before quickly folding into the egg whites with a spatula or metal spoon.
In a third mixing bowl, sift together the flour and cornflour. Add it to the egg mixture along with the vanilla seeds and lemon zest, cutting and folding until incorporated. Spoon the batter into the baking tin and rap it gently on a flat surface to help settle it.
Bake until risen and dry to the touch, about 35-45 minutes; when ready, a toothpick should come out clean from the middle.
Remove the cake from the oven and leave to cool slightly before turning out onto a wire rack to finish cooling. Serve warm or at room temperature.