Place bread cubes on a baking sheet in a single layer and toast in a 400 degree F oven for 5-7 minutes or until crisp. Reserve ½ cup cubes, place the remainder in the bottom of a greases two-quart casserole dish.
In a medium bowl, beat eggs and green onion. Scramble mixture in a non-stick skillet over medium heat until soft curds form. Layer scrambled eggs over bread cubes.
In a heavy saucepan, melt butter. Stir in flour, salt, pepper, and milk. Cook, stirring constantly over medium heat until thick and bubbly. Remove from heat. Stir in mustard and cheese until cheese is melted.
Lay strips of smoked salmon over the eggs, and then pour sauce over all. Top with reserve bread cubes. Bake in a 350 degree F oven for 20 minutes or until heated through.