Ree Drummond's The Pioneer Woman website features an easy spinach salad dressing made from bacon, red onions, vinegar, mushrooms, Dijon mustard and salt. It is simple recipe that home cooks can prepare in less than 30 minutes.
Start the dressing by cutting eight slices of thick-cut bacon in half and frying them over medium heat until crisp. Remove the bacon, drain on paper towels, and allow to cool. Once bacon has cooled, crumble it and set it aside. Reserve 3 tablespoons of rendered fat from the pan for the dressing, and move 2 additional tablespoons to a new skillet. Slice one small, whole red onion very thinly, add the slices to the skillet and cook them over low heat until reduced and caramelized. Remove them from the pan, and set aside. Thinly slice one package of white button mushrooms and add them to the pan, sautéing until slightly browned. Set browned mushrooms aside with the onions. Add the 3 tablespoons of reserved bacon fat to the pan. Over medium low heat, whisk in 2 teaspoons of sugar, 3 tablespoons red wine vinegar, 1/2 teaspoon Dijon mustard and a dash of salt. Once you've combined the ingredients, add spinach, bacon crumbles, onions and mushrooms to a bowl, and toss them with the still-warm dressing. Finish the salad with sliced hard-boiled eggs if desired.