Trim the spinach of coarse stems and blemished leaves, and wash. Pat the spinach dry with paper toweling, tear into bite-size pieces, and put into a large serving bowl along with the mushrooms and onion.
Set aside. Meanwhile, cook the bacon in a 12 inch skillet over moderately high heat until crisp -- about 5 minutes. Drain on paper toweling and reserve. Pour off all but 5 tablespoons of the drippings. Reduce the heat to low. Add the mustard, sugar, vinegar, Worcestershire sauce, and lemon juice to the drippings. Stir to mix well. Pour the dressing over the salad, crumble the bacon over it, and toss thoroughly. Serve immediately.
260 calories per serving.
Note: To make ahead, prepare the salad and dressing, but do not combine. Cover the salad with plastic food wrap and refrigerate.
Before serving, heat dressing and toss with the salad.