Thinly slice beets, carrots, radishes, fennel bulb (trim the fronds and reserve them for later), and red onion on a mandoline slicer. (To keep beets from turning everything pink, set shaved beets in a separate bowl and wash down the mandoline before shaving other vegetables.) Drizzle shaved beets with olive oil and a pinch of sea salt, and toss. Place remaining shaved vegetables in a large bowl, sprinkle with sea salt, and toss.
To make the dressing, combine lemon juice, minced garlic and red onion, and a generous pinch of sea salt in a small mixing bowl. Whisk in tahini, and then the olive oil. Add black pepper and additional sea salt to taste.
Cut beet greens and bok choy into thin ribbons. Core apple and slice very thin.
Drizzle the beets with several tablespoons dressing. Do the same for the vegetables. In a large salad bowl, drizzle beet greens, bok choy, and apple with dressing. Just before serving, toss everything but the beets with the greens. Tuck beets throughout. Finish with a few drizzles of dressing and a pinch each of sea salt and black pepper. Garnish with reserved fennel fronds.
To make ahead, prep vegetables and make dressing. Day of, add sea salt to shaved vegetables, and sea salt and olive oil to beets. Just before serving, toss each element with dressing and finish as above.