Mix together the eggs, milk, mustard and salt; pour over the bread, ham and cheese mixture.
Melt the butter and pour this over the top. Cover and refrigerate overnight.
Uncover in the morning and bake at 325 degrees for 1 hour.
SLOW COOKER VARIATION: Mix up several hours before, if possible, and keep in the fridge. Start on low at midnight and it will be ready between 7-8 a.m.