Preheat oven to 400 degrees F. Wash beet well, then wrap in foil and roast in oven 1 hour, or until easily pierced with knife. Cool, then wrap in paper towel and rub to remove skin. Quarter beet and slice each quarter into 8 thin slices from the root end to the stem end; set aside. Quarter fennel bulb; remove core. Slice each quarter very thinly and set aside. Using a sharp knife, completely remove the skin of the 2 oranges and cut into segments over a bowl, reserving any juice.
For the vinaigrette: Place grated citrus zests in a bowl with mustard, vinegar, salt, pepper, and 1⁄4 cup reserved orange juice. Whisk, incorporating oil slowly. Check seasoning and adjust as desired.
To assemble the salad: Mix greens together with orange segments, beets, and fennel; toss with 1⁄2 cup vinaigrette. Taste and add more vinaigrette as desired. Divide salad mixture evenly among 4 plates. Top with goat cheese and serve.