• Heat oven to 375°F/190°C.

Make the Crumble

• In medium bowl, mix together the flour, and sugar. Cut the cold butter into small pieces and add it to the flour.  Using both hands, rub the butter into the flour and sugar until it looks like large breadcrumbs.  You can also use a pastry blender or two knives to do this step, if you prefer. Set the bowl aside while you prepare the apples.

Prepare the Apples

• Peel and core the apples and chop into big pieces. 
• Place the apples into your dish and sprinkle 2 tablespoons of sugar over the top.  Give the apples a quick stir so the sugar gets incorporated.
• Sprinkle the crumb over the apples and put dish into the oven and bake for 45 minutes until the top starts to turn golden brown and the apples are bubbling happily away in their juices.

Make the Custard

• Begin by warming the milk in a medium pot over low heat.  Remove the milk from the heat once it begins to simmer.
• Next, separate the egg yolks from the whites.  Keep the whites to use in other recipes.  Add the yolks to a small bowl along with the cornstarch (or cornflour) and sugar and whisk until pale.
• Add some of the hot milk into the egg mixture a little bit at a time and stir continuously.  After you have added a couple of spoonfuls of milk to the eggs, pour the entire egg mixture into the pot with the milk and place back on the heat.  Add a teaspoon of vanilla. At this point, you are going to stir with a wooden spoon continuously while the custard cooks.  It will take about 20 minutes over a low heat to thicken.  You will know it is ready when it coats the back of a spoon and you can run your finger down the middle while leaving a trail.
• Strain the custard through a fine sieve to strain out any lumps and catch any egg that might have scrambled.
• Serve the apple crumble warm with a big drizzle of warm custard over the top.