Place beets in a 5-quart saucepan with enough water to cover.  Boil for 45 minutes.  Immediately drain, fill saucepan with cold water for the beets to cool.  Drain one more time and set beets aside to cool completely.  Remove peel by rubbing with a paper towel or a pairing knife.  Slice in wedges.

Combine vinegar, oil, salt and pepper in a small bowl.  Whisk together with a small whisk.  Set aside.

Arrange spinach, mint leaves, beet and plum wedges on a large serving dish.  Toss with dressing and top with feta and walnuts.  Enjoy!