Cover bottom of buttered 9x13 inch baking pan with half of bread cubes. Layer broccoli, ham and cheese over bread; top with remaining bread cubes. Combine eggs, milk, onion, salt and mustard; pour over strata. Refrigerate, covered, several hours or overnight. Bake, uncovered, in a preheated 350 degree oven until tests done when knife inserted in center comes out clean (55-60 minutes). Let stand 10 minutes. Cut into squares.