Trim off each end of the beets and using tin foil, loosely wrap each beet individually and place on a baking sheet. Roast for 1 hour, remove from oven and allow to cool to room temperature. Peel off the skin of each beet using your fingers (gloves are helpful!) or a paring knife. Slice each beet and refrigerate until ready to use.
Toss the spring mix with the balsamic vinegar and olive oil until well-coated. Top with walnuts, goat cheese, and roasted beets. Serve immediately.