Step 1Heat the oven to 350 degrees. Rinse the beets and place them in a small casserole with a cover. Add about one-half inch water and roast until the skin easily slips from the beets. (Baby beets will take approximately 20 minutes; larger beets may take an hour or longer.) Remove the beets from the oven and, while still warm, remove the skin. Cut into one-fourth-inch-thick slices. Set aside. (This step can be done the day before -- the beets will actually grow sweeter if left in the refrigerator overnight.)
Step 2Place the onion in a nonreactive bowl. Sprinkle with one-fourth teaspoon salt and the vinegar and stir to coat. Let rest 1 hour.
Step 3Heat 1 teaspoon olive oil in a medium skillet. Add the pancetta and cook over low heat until it starts to turn crisp, about 5 to 7 minutes. Drain on paper towels and cool slightly. Cut into one-half-inch pieces and set aside.
Step 4Combine the beets, onions and greens in a salad bowl. Toss with the remaining one-fourth teaspoon salt, one-fourth teaspoon pepper and the remaining 2 tablespoons olive oil to lightly coat. Taste; adjust seasoning. Divide among 4 plates and garnish with the pancetta.