In a large skillet, cook bacon until crisp; remove bacon and set aside. Drain, reserving 2 tablespoons drippings in the skillet. Add onion and saute until tender; remove from the heat. Add the vinegar, water, sugar, pimientos, mustard, salt, pepper and bacon; mix well.
Combine cornstarch and cold water; stir into skillet. Cook and stir until mixture comes to a boil. Boil for 2 minutes, stirring constantly.
Serve warm over fresh spinach or mixed greens. Refrigerate leftovers and reheat before serving.