Preheat oven to 400°F. Wash and scrub beets, then wrap them in aluminum foil and roast them for about45-60 minutes, or until they are tender when pierced with a fork. Once cool, peel and slice the beets.
In a large bowl, combine sliced beets, sliced oranges, crumbled feta cheese, and shelled pistachios.
In a small bowl, whisk together olive oil, balsamic vinegar, honey, salt, and pepper to make the dressing.
Add baby spinach or mixed greens to the bowl with the beet and orange mixture, then pour the dressingover the top and toss gently to coat.