For the best flavor and texture, use freshly cooked beets. Roasting the beets will bring out their natural sweetness. A mandoline slicer makes quick work of thinly slicing the fennel. Be sure to use the hand guard for safety! If you prefer a sweeter salad, add a drizzle of honey or maple syrup to the dressing. This salad can be prepared ahead of time, but wait to add the dressing until just before serving to prevent the fennel from becoming soggy. Consider adding toasted nuts, such as walnuts or pecans, for extra crunch and flavor.