I. Prepare the Filling:

1. In a large bowl, combine sliced apples, brown sugar, cinnamon, nutmeg, lemon juice, and flour. Mix well to coat the apples evenly.

II. Assemble the Pie:

2. Preheat your oven to 425°F (220°C).
3. Roll out the pie crust and place it in a 9-inch pie dish. Pour the apple mixture into the crust, spreading evenly.
4. Sprinkle the toffee bits over the apples.
5. Cover with a top crust (or lattice) and crimp the edges to seal. Brush the top with beaten egg.
6. Cut slits in the top crust to allow steam to escape.

III. Bake the Pie:

7. Bake in the preheated oven for 15 minutes. Reduce the temperature to 350°F (175°C) and continue baking for an additional 30-35 minutes, or until the apples are tender and the crust is golden brown.

IV. Make the Custard:

8. In a saucepan over medium heat, combine the milk, sugar, and salt. Heat until just below boiling.
9. In a separate bowl, whisk the egg yolks and cornstarch together. Gradually add the hot milk mixture to the yolks, whisking constantly.
10. Return the mixture to the saucepan and cook over low heat, stirring continuously until thickened (about 5-7 minutes).
11. Remove from heat and stir in vanilla extract. Let cool slightly.

V. Serve:

12. Once the pie is baked, let it cool for a few minutes. Serve slices warm with a generous drizzle of vanilla custard and additional toffee bits on top.