Whisk together in a small bowl:
      1/4 cup cider vinegar
      1/4 cup sour cream
      1/4 cup buttermilk
      3 tablespoons honey
      1 teaspoon minced garlic
      1 scallion, minced
      Pinch of ground red pepper
      Salt and ground black pepper to taste
Add in a slow, steady stream, whisking constantly:
      1/2 cup olive oil
Combine in a salad bowl:
      4 cups bite-sized pieces arugula, washed and dried
      1 small head radicchio, washed, dried, and torn into bite-sized pieces
      2 Belgian endives, washed, dried, and sliced lengthwise into long strips
Stir the dressing well, add just enough to moisten the greens, and toss to coat. Divide the greens among salad plates and top with:
      2 Granny Smith apples, cored and very thinly sliced or other tart apples
      1/2 cup pecan halves, toasted
Serve immediately.