Combine the first 4 ingredients in a bowl; stir in the bacon, green onion and salt and pepper. Cover and refrigerate until needed. Soak the spinach in very cold water a few minutes to liven it up. Drain well and then dry on a towel. Divide among plates and top with the mushrooms and onions; serve the dressing alongside for spooning on top.
Options: Use another green in the salad, such as chopped romaine, baby mixed greens or chopped leaf lettuce. Use brown mushrooms instead of white, if you prefer their earthier flavour. Give the dressing an herbaceous taste by also mixing 2 to 3 tsp. of chopped fresh dill, tarragon or chive.