Preheat your oven to 350°F (175°C). Gather all your ingredients and equipment.

Cut your white bread into cubes, then spread it on a baking sheet. Bake for 10 minutes to dry out.

In a large bowl, whisk together eggs, half and half, and Dijon mustard. Allow bread cubes to cool, then mix them in.

Grease a baking dish with butter. Layer half of the soaked bread mixture, then add ham and half of the Gruyere cheese.

For the béchamel, melt butter in a saucepan, whisk in flour, then add milk gradually, cooking until thick. Add nutmeg.

Pour béchamel over casserole, top with remaining Gruyere cheese, and bake for 30 minutes.

Switch to broil for 5-10 minutes until golden brown. Watch closely to prevent burning.

Let cool for a few minutes, then slice and serve hot.