For an enhanced flavor, consider using freshly squeezed lime juice. Toasting the walnuts brings out their nutty aroma and adds a delightful crunch. If you don't have walnuts, pecans or almonds are a good substitute. The salad is best served immediately after preparation to prevent the apple from becoming too soft. However, you can prepare the vinaigrette and slice the vegetables ahead of time. A mandoline makes quick work of slicing the fennel and apple, ensuring even and thin slices.