Line a greased 9×13 pan with bread, trimming to fit. Sprinkle cheese over top. Scatter sausage over cheese. Beat eggs together well. Add first amount of milk, mustard, onion salt and pepper. Mix. Pour over cheese-sausage mixture. Chill overnight. About 1 3/4 hours before serving, mix mushroom soup with second amount of milk pour over all. Melt margarine. Stir in crumbs. Sprinkle over soup mixture. Bake, uncovered, in 350 F oven for about 1 1/2 hours.