Puree cranberries in processor until smooth. Add vinegar, onion, sugar and mustard and process until well blended.
With processor running, gradually add oil and process until well blended.
Transfer to medium bowl. Season to taste with salt and pepper.
Dressing can be made one day ahead of time and refrigerated. Bring to room temperature and whisk before using.
Combine greens and apples in a large bowl. Toss with enough dressing to coat and sprinkle with walnuts.
Serve, passing remaining dressing separately.
Note1: I like Granny Smiths or Jonagold apples in this, although the original recipe calls for red delicious. I sometimes dice the apples, rather than slicing.
Note2: This is an older recipe, and I use much less oil in it these days - maybe 2/3 cup? I just pour it in and taste until it tastes right. I use about 2/3 grapeseed or other neutral oil and 1/3 olive oil.