Place 1/3 of the roasted beets into a blender or food processor with the stock. Puree until smooth (the mixture will be very liquidy), then transfer to a saucepan and bring to a gentle simmer. Reserve remaining beet wedges.
Place a rack in the center of oven and preheat to 400 degrees F. Cut off the beet leaves near the tip of the beet (save the beet greens for another use) and scrub the beets well. Toss beets with 2 tablespoons olive oil, then wrap in tin foil (small beets can be wrapped together; large beets should be wrapped separately). Place beets on a baking sheet to catch any juices, then roast for 50-60 minutes, until fork tender (time will vary depending upon the size of the beets.) Check the beets every 20 minutes to ensure they are not burning on the bottom. If they begin to scorch, drizzle with a little water, then rewrap and continue cooking. Remove from oven and set aside until cool enough to handle. Slip off beat skins and slice into 1/2-inch thick wedges. (At the point, the beets can be refrigerated in an airtight container for up to 1 week.)