In a large skillet, cook bacon over moderate heat, turning it, until it is crisp, transfer it to paper towel lined plate to drain, and discard all but 2 tablespoons of the fat. In the remaining fat, cook the shallot for 1 minute. Add the vinegar, sugar, mustard, oil, and salt and pepper to taste. Bring to a boil, pour over spinach and toss. Serve sprinkled with the bacon.