Put egg in bottom of bowl and beat with fork. Add mayonnaise and blend. Add soup and milk. Cook spinach and drain but leave in pan. Add cheese so that it melts. Fold in bowl with egg mixture. Put in buttered casserole. Top with 1/4 cup buttered bread crumbs. Bake at 350 for 45 minutes. If you double recipe, put in 9x13 pan.