Rub in the butter gently with your fingertips until the mixture resembles coarse breadcrumbs. Bring the crumbly dough together into a ball.
Wrap the streusel dough in cling film or a plastic bag and chill in the refrigerator for 30 minutes.
Pre-heat oven to 190 deg C (375 deg F). Grease a 20-cm or 8-inch loose-bottomed cake tin with butter and dust lightly with flour. Shake off the excess flour.
In an electric mixer, with a paddle attachment fixed on, place butter and sugar in the mixer bowl.
Cream butter and sugar on medium speed (between Speed 3 to 4 on my Kitchen Aid mixer).
Cream until the batter turns pale and creamy in colour, and is light and fluffy.
Add the lemon zest and cream with butter on low speed (speed 2 on my Kitchen Aid mixer) until well dispersed through the batter.
Beat in the eggs, a little at a time, mixing well after each addition to avoid curdling.
Beat until the eggs are well incorporated in the batter.
Sift the flour into the batter and gently fold with a metal spoon or spatula.
Finally, add the milk to the batter and gently mix it in until well incorporated.
Spoon half of the batter into the prepared tin and spread evenly. Smooth the surface with a palette knife.
Arrange half the apple wedges over the batter, reserving the best looking pieces for the top. Use larger apple wedges to line the circumference.
Fill the centre with the smaller apple wedges.
Spread the rest of the batter over the apples. Smooth once more with a palette knife. Arrange the remaining apple wedges on top of the cake in an attractive pattern.
Remove the streusel dough from the refrigerator and coarsely grate it.
Sprinkle the grated streusel evenly over the top of the cake.
Bake in the centre of the oven for 45 minutes. Test with a skewer by inserting it into the centre of the cake. If the skewer emerges coated in batter, cook for a few minutes more and test again. When baking is finished, remove the cake from the oven and leave the cake in the tin for 10 minutes.
Keeping the streusel on top, carefully remove the cake from the tin and cool on a wire rack. Serve warm.