Preheat oven to 425°F. In a 9×13-inch baking dish, pour salt half-inch deep. Wash beets. While wet, roll beets in salt to coat, then place on the salt bed. Cover dish with foil and roast for 50 minutes or until beets are fork tender. Remove from oven, discard foil and cool beets in salt. Once cool, remove skin from beets and slice into ¼-inch rounds. Reserve. You may reserve the salt in a ziplock bag for another use.
While the beets are cooking, remove the outer paper from the shallots and slice the root off of each bulb. Toss shallots in oil, wrap loosely in a foil packet and place directly in the oven. Roast for 30 minutes in a 425°F oven. Remove from the oven and allow to cool. Once cool, slice shallots in half lengthwise and peel into petals. Reserve.
For the toasted walnuts, decrease the oven to 350°F. Place walnuts on a rimmed baking sheet (heavier baking sheet, if possible). Bake 5-10 minutes, shaking and turning half-way through. Remove from the oven and reserve.
In a large bowl, lightly season mizuna, arugula and mustard greens with salt and pepper, then gently toss with 3 tablespoons of sherry shallot vinaigrette. Divide greens between servings and plate. Top with beets, shallot petals, walnuts and pomegranate seeds.