In a large skillet cook the bacon over moderate heat, turning it, until it is crisp, transfer it to paper towels to drain, and discard all but 2 tablespoons of the fat. In the fat remaining in the skillet cook the shallot and the apple over moderate heat, stirring, for 1 minute, add the vinegar, the cider, and salt and pepper to taste, and boil the mixture, stirring occasionally, for 8 to 10 minutes, or until it is reduced to about 1/2 cup. Whisk in the mustard, the oil, and salt and pepper to taste. In a large bowl toss the spinach with the warm dressing until it is just wilted and sprinkle the salad with the bacon. Serves 4. Gourmet October 1991