Preheat the oven to 190°C conventional / 170°C fan. Grease a loaf tin measuring approximately 23 × 13 cm or 25 × 13 cm and line it with baking paper.
2. Place the butter, milk and vanilla bean paste or extract in a small saucepan. Heat gently until the butter has melted. Remove from the heat and let cool slightly.
3. In a medium bowl, whisk together the flour, baking powder, orange zest, salt and nutmeg.
4. Using a stand mixer with a whisk attachment or a hand mixer, beat the 100 g sugar and eggs on medium-high speed for 1–2 minutes, until pale and fluffy. Scrape down the sides as needed.
5. Add half the dry ingredients and mix on low speed until just combined. Slowly pour in the milk mixture, then add the remaining dry ingredients. Mix on low speed until smooth.
6. Peel, core and quarter the apples. Using a mandoline or a very sharp knife, cut them into thin slices, approximately 1.5 mm thick. Gently fold the slices into the batter with a spatula until evenly coated. Slice and coat one apple at a time to help prevent browning.
7. Spoon the mixture into the prepared tin in batches, flattening the apple slices after each addition. Arrange the top layer neatly if desired. Gently tap the tin on the worktop to release trapped air, then sprinkle with 1 tbsp sugar.
8. Bake for 60–70 minutes, until golden.
9. Place the tin on a wire rack and let the cake cool in the tin for 1 hour.
10. Run a knife around any exposed edges, then carefully lift the cake out. Let it cool on the rack for another hour.
11. If desired, dust with icing sugar or serve with whipped cream. Slice with a serrated knife and serve at room temperature or chilled.