Combine spinach, mushroom, and onions. Set aside. Cook bacon in skillet until crisp, put on paper towel to drain. In skillet with 2 tablespoons bacon drippings, add vinegar, sugar, and dry mustard. Bring to a boil, then remove from heat. Add spinach mixture and toss in hot skillet for 30 -50 seconds until spinach wilts. Sprinkle on bacon and chopped boiled egg. Toss and serve immediately.