Preheat oven to 170°C / 340°F and grease or line a loaf tin.
Combine custard and flour until smooth.
Add lemon zest, juice and optional sugar. Fold gently until combined. Do not overmix.
The batter should be thick and spoonable, not pourable. It should drop from a spoon but still need smoothing in the tin. If runny, your custard may be too thin. If stiff, your custard may be too thick.
Spoon into prepared tin, smooth the top and sprinkle with flaked almonds.
Bake for approximately 40 minutes or until golden, toasted on top and a skewer comes out clean.
Cool in tin for 10–15 minutes before transferring to a wire rack. Cool before slicing.
