Line a 24 cm round cake pan (with a removable bottom) with foil, grease.
To make crust: cream butter and sugar; add egg, mix to combine; add flour, ground almonds and baking powder, quickly mix into batter. Spread batter on bottom of the prepared pan.
Peel, core and slice apples. Cover the surface of the batter with apple slices.
Bake in the preheated oven for 15-20 minutes.
While the pie is baking, prepare the topping: Combine all topping ingredients in a saucepan. Heat the mixture gently to boiling point, stirring occasionally. Remove from heat as soon as the first few bubbles appear. Do not boil!
Take the half-baked pie out of the oven and quickly spread the hot topping on top of it.
Place the pie back in the oven and continue baking for about 20 minutes, or until the topping is golden brown.
Let cool completely before removing from pan. You can also use a 24 cm pie dish and serve the pie in its dish.