1. Heat the oven to 350°. Toast the walnuts until golden brown, about 8 minutes. Let cool.
2. In a large frying pan, cook the bacon until it is crisp. Drain the bacon on paper towels and then crumble it.
3. In a small glass or stainless-steel bowl, whisk the vinegar with the mustard, salt, and pepper. Whisk in the oil.
4. In a large bowl, combine 2 tablespoons of the dressing with the chicken. Let sit for about 5 minutes so that the chicken absorbs the dressing. Add the walnuts, bacon, spinach, onion, apple, and the remaining dressing and toss.
Menu Suggestions: Hot garlic bread, served either plain or with a little Parmesan cheese, goes well with all the flavors here.
Wine Recommendation: This substantial fall salad, with its hearty flavors, will taste great with a Beaujolais or, for something off the beaten path, a fruity pinotage from South Africa. In either case, chill the bottle for fifteen minutes or so before serving.