Prepare the Shortcrust Pastry:

1. Sift the flour and baking powder onto a clean surface and make a well in the center.
2. Add sugar, salt, and grated lemon peel into the well.
3. Beat the egg and pour it into the well, mixing lightly with sugar using your fingers.
4. Incorporate softened butter and blend until the mixture resembles crumbs.
5. Knead until forming a smooth dough, roll it between parchment papers, and refrigerate for at least 2 hours.

Make the Custard:

1. Heat the milk and lemon peel to just before boiling.
2. In a separate bowl, whisk eggs, sugar, and flour.
3. Remove lemon peel from milk and slowly incorporate into the egg mixture.
4. Return to heat and stir constantly until the custard thickens. Once thick, remove from heat, cover with cling film directly on the surface, and cool.

Assemble and Bake the Tart:

1. Preheat the oven to 356°F (180°C) and grease a 24 cm round baking tin.
2. Roll out the pastry to fit the tin, trim excess, and prick the bottom with a fork.
3. Spread apricot jam over the base, followed by a thick layer of custard. Sprinkle with cinnamon.
4. Peel, core, and slice the apples. Arrange them on the tart and sprinkle generously with sugar.
5. Bake for about 50 minutes until golden. Allow to cool before serving.