Heat oven to 325°. Spread salt in the bottom of a small baking dish. Arrange beets on top and cover tightly with aluminum foil. Roast until tender when pierced with a knife, about 1½ hours. Let cool enough to handle and then peel. Cut into wedges or thinly slice.
Combine sherry vinegar, mustard, brown sugar, shallot, and garlic in a small bowl. Season with salt and pepper. Slowly add oil, whisking constantly, until emulsified.
Toss mixed greens with enough dressing to moisten. Arrange dressed greens on a plate, add beets and top with goat cheese, candied nuts, and baby arugula. Just before serving, drizzle salad with Truffle Honey.
For the Truffle Honey: Blend ¼ cup orange blossom honey with 1 tablespoon white truffle oil.