Peel the beets and the shallot. Combine them in a food processor fitted with the metal blade and pulse carefully until the beets are shredded; do not purée. (Or grate the beets by hand and mince the shallots; combine.) Scrape into a bowl.
Shave the fennel very thinly, and add it to the bowl with the beets. Sprinkle with salt and pepper, then add the mustard (if you’re using it), oil, and lemon juice and toss. Taste and adjust the seasoning. Toss in the herbs and serve.