1. Preheat oven to 350ºF. Spread walnut pieces on a rimmed baking sheet. Bake walnuts for 5 to 8 minutes, stirring every 2 minutes, until just toasted. Pour into a bowl to cool.
2. Place apples in a bowl and toss with 1 Tbsp. vinegar. In a small bowl, whisk together remaining vinegar, mustard, shallot and sugar; season with salt and pepper. Whisking constantly, drizzle in olive oil and continue to whisk until smoothly blended.
3. Place salad greens, apples and walnuts in a large bowl. Toss with dressing just before serving.
This recipe also appeared in the October 2009 issue.