Preheat your oven to 400°F. Wrap the beets in aluminum foil and roast for about 45-60 minutes, or until tender. Allow to cool, then peel and dice.
In a large salad bowl, combine the fresh spinach, diced beets, mandarin orange segments, and sliced red onion.
In a small bowl, whisk together the olive oil, balsamic vinegar, salt, and pepper to create the dressing.
Drizzle the dressing over the salad and toss gently to combine.
Sprinkle the toasted pine nuts and crumbled feta cheese on top before serving.