In a small bowl, stir the goat cheese, vinegar, mustard and sugar until smooth. Stir in the buttermilk and 2 tablespoons of water until smooth; if necessary, stir in more water. Season with salt and white pepper. Stir in the scallion just before serving.
Make Ahead: The dressing can be refrigerated, covered, for up to 3 days.
Notes: ONE TABLESPOON: Calories 16 kcal, Protein 1 gm, Carbohydrate .3 gm, Cholesterol 4 mg, Total Fat 1.3 gm, Saturated Fat .9 gm