The beets can be roasted up to 48 hours before plating the salad. The dressing can be made 48 hours ahead and the fennel can be cut and soaked up to 12 hours ahead.
To toast fennel seeds, place them in a small sauté pan set over high heat. Move the pan briskly back and forth to agitate the seeds so they won’t burn. Cook for 3 – 4 minutes to bring out the flavor. Remove the seeds to a small bowl or baking sheet to cool.