Preheat the oven to 200°C/ 400°F/Gas mark 6. Grease a large baking dish with butter.
Butter the bread, remove the crusts and cut into quarters diagonally. Line the sides of the baking dish with the bread, overlapping slightly.
In a large bowl, mix together the eggs, cheese, milk, onion, pepper and mustard. Season with salt and pepper, and pour over the bread.
Cook in the oven for 20–25 minutes. Serve either hot or cold.