To Roast the Beets: Adjust the oven rack to the center position and preheat the oven to 400°F Rub the beets with the olive oil and wrap them together in a large piece of aluminum foil. Put the beets in a small roasting pan or baking dish and roast them for 50 minutes to an hour, until they are tender when a knife is inserted in the center.
Remove the foil packet from the pan and let the beets cool on the foil before unwrapping. Using your fingers, peel the beets over the foil. The skins should slip off easily; if not, use a paring knife. Cut the beets into 1/4-inch matchstick slices and reserve.
To make the Vinaigrette: Put the olive oil, balsamic vinegar, red wine vinegar, onion, jam, mustard, and sugar in the container of a blender or food processor. Process until well blended and emulsified. (You should have 1 cup.) Season to taste with salt and pepper.
To Assemble the salad: In a large salad bowl, toss the greens, beets, and walnuts with half of the dressing. Season to taste with pepper. Top each serving with some of the cheese. Pass the remaining dressing at the table.