In a large bowl combine spinach, mushrooms, and green onions. If desired, sprinkle with pepper; set aside.
For dressing, in a 12-inch skillet cook bacon until crisp. Remove bacon, reserving 2 tablespoons drippings in skillet (add vegetable oil, if necessary). Or, if desired, substitute 2 tablespoons vegetable oil for bacon drippings. Crumble bacon; set aside. Stir vinegar, sugar, and dry mustard into drippings. Bring to boiling; remove from heat. Add the spinach mixture. Toss mixture in skillet for 30 to 60 seconds or until spinach is just wilted.
Transfer spinach mixture to a serving dish. Add chopped egg and crumbled bacon; toss to combine. Serve salad immediately.