Preheat the oven to 190°C (gas mark 5) and lightly grease a 23cm x 33cm ovenproof dish.
In the prepared dish, combine the chicken, ham and 100g of the grated cheese, stirring to distribute evenly.
To make the sauce, melt 30g of butter in a saucepan over medium heat. Add the flour and cook for 1-2 minutes, stirring. Gradually whisk in the milk, crumbling in the chicken stock cube, then bring to a simmer and cook until thickened. Stir in the mustard and pepper.
Pour the sauce over the chicken and ham mixture.
Sprinkle the remaining 50g of cheese evenly over the top.
In a small bowl, toss the breadcrumbs with the melted butter until coated, then scatter over the cheese.
Bake for 25-30 minutes, until bubbling around the edges and the topping is golden and crisp.
Remove from the oven and leave to rest for 5 minutes before serving.