Trim the stems, root end and any tough outer leaves from the fennel. Slice it in half lengthwise. Then place the halves, cut side down on a cutting board and thinly slice. Core the apples and cut them into thin wedges. On 4 individual salad plates, arrange overlapping slices of fennel and apples in a sunburst pattern. Thin the salad dressing with the vinegar. Then spoon or drizzle the dressing over the fennel and apples. Sprinkle on the pecans and serve.