1. Preheat the oven to 200°C/gas mark 6. Generously butter an ovenproof dish, about 30 by 25 cm. Use the sliced bread, remaining butter, mustard, ham and cheese slices to make sandwiches, then cut in half diagonally.
2. Arrange the sandwich triangles in the dish, layering slightly over each other. Beat 5 of the eggs with the milk until you have a batter. Season and whisk in the chives. Gradually pour over the sandwiches, making sure they are well soaked through. Sprinkle over the cheddar cheese. Finally, crack the remaining 7 eggs on top of the sandwiches, spacing out evenly. Bake for 20 minutes until golden and then eggs are cooked through. Serve with a crisp green salad.