1. Start with the vanilla custard. Mix the cornstarch with sugar, vanilla extract, salt, and several tablespoons of the milk in a small bowl until well combined. Set aside.

2. Add the milk to a small pot and bring to a boil while stirring constantly. As soon as you can see bubbles coming stir in the cornstarch mixture – it will thicken up very quickly. Let cook for a moment, then remove from the heat. Let cool down for 2-3 minutes, stirring often, then add the egg and mix in well. Place a piece of plastic wrap directly on the custard to avoid skin from forming and let cool down.

3. For the streusels mix the flour with sugar, cinnamon, nutmeg, and salt in a bowl. Melt the butter and add it together with the vanilla extract to the bowl. Mix to combine, then add the rolled oats and mix everything with your fingers to get a nice streusel mixture. Place in the fridge until needed.

4. Preheat the oven to 350°F (180°C). Line an 8 inches (20cm) springform tin with some baking parchment and grease lightly. Fill about half of the streusel mixture into the tin and press onto the bottom to create an even layer. Peel, core, and cut the apples into dice. Add to the bowl with the remaining streusels and mix to combine. Pour the custard into the tin and smooth out the top. Distribute the streusel mixture evenly on top, sprinkle with some brown sugar (optional) and bake for about 35-40 minutes. Take out of the oven and let cool down completely in the tin, then place in the fridge for at least 2 hours (or overnight) before serving.