Cut bread into small cubes and set aside.
Melt margarine in uncovered frying pan on stove top; saute onions until soft; stir in mustard.
Spread 1/3 of the cubed bread on the bottom of a 10-cup casserole dish (a 13x9x2 rectangular pan is equal to a 10-cup dish). Drizzle 1/3 of the margarine/onion/mustard mixture evenly over the bread; top with 1/2 of the ham and 1/2 of the cheese.
Repeat layering – bread, margarine mixture, ham, cheese.
Finish layering with bread and margarine mixture.
In a large bowl, beat eggs with wire whisk; add milk and blend well.
Pour egg mixture evenly over casserole, pushing down the bread as needed to soak in the egg mixture.
Cover casserole and refrigerate overnight.
Bake uncovered in a 325 degree oven for 1 hour, or until golden brown and puffy.