Preheat oven to 400 degrees.
Wrap beets individually in aluminum foil and place on a sheet pan. Roast for 50 minutes to 1 hour, depending on size, until sharp knife inserted in middle pushes in easily.
Unwrap each beet and let cool for 10 minutes. Peel beets.
Whisk together both vinegars, olive oil, mustard, 1 tsp. salt, and 1/2 tsp. of pepper and set aside. While beets are warm, cut each one into wedges, then cut each wedge in half and place in mixing bowl. While still warm, toss with 1/2 of the vinegar dressing. Add salt and pepper to taste.
Place the arugula in a small bowl and drizzle with the other half of the dressing. Toss, then place on individual plates. Arrange the beets over the arugula, the sprinkle with pumpkin seeds, and feta cheese.
This recipe was adapted from my all-time favorite cook, Ina Garten--The Barefoot Contessa. Her version can be found on the Barefoot Contessa website under Balsamic Roasted Beet Salad.