Wrap beets in foil and roast until tender, about 45 minutes
Allow beets to cool enough to handle, then use the foil to rub off the skins. Cut beets into wedges. Set aside to cool completely.
In a large bowl, whisk oil, vinegar, thyme, and salt. Add beets, shallots, and orange segments, tossing to coat.
Put the salad in the fridge to marinate, at least two hours, but overnight is best.
Before serving, toss with pistachios and parsley
Serve on a bed of arugula
Notes
Clementines or mandarin oranges are easy to peel and segment, and they are just the right size for salads. But if you can't find them, any orange will taste just as delicious.