Cook the beets. Start by washing the beets well, then toss with teaspoon of olive oil and 1/2 teaspoon salt. To steam-roast, put beets in small to medium sized baking dish. Pour in water to about ½ inch. Cover dish tightly with foil. Cook in the oven at 425° F until beets are very tender, about one hour. (Cooking time for individual beets is affected by their size and can vary quite a bit, with larger beets taking longer and smaller beets cooking faster. )
Check beets for doneness. Be careful to avoid released steam when peeling foil away from baking dish. The beets are done when a thin bladed knife inserted in the center meets very little resistance. Alternatively test beets by piercing beets through the foil with paring knife. Smaller beets may cook faster than larger ones, remove as needed.
When beets are done, allow them to cool. Then remove skin using a paring knife, it will slip off easily. Cut beets into wedges (or preferred shape).
Prepare the oranges. Start by cutting a thin slice off the top and bottom. Use a sharp paring knife to remove the rind and pith. Then slice into wedges by cutting each orange segment lengthwise. (You could get fancy and supreme the oranges, but I prefer my faux supreme method, for ease and a bit more fiber.) Thinly slice or dice the red onion.
Chop the fresh mint.
Make the dressing. Combine the balsamic vinegar, mustard, sugar, olive oil, 1/8 teaspoon salt, and 1/8 teaspoon pepper in a small mason jar. Shake well. (Alternatively, whisk thoroughly in small bowl.) Taste and add more salt and pepper as desired.
Assemble the salad. Combine beets, oranges, onions, and mint in medium bowl. Pour in the dressing. Stir gently until well mixed. Serve immediately or refrigerate for several hours to allow flavors to come together. If desired, garnish with additional fresh mint leaves for serving.