Wrap beets in aluminum foil, place on a baking sheet, and roast in the oven until cooked through, approximately 45 to 60 minutes - depending on beet size. When beets are fork tender, remove from the oven and set aside.
When the beets are cool enough to handle, slip off the skins and cut the beets into a 1/4-inch dice. You should have about 6 cups of sliced beets. Set aside.
In a medium-sized pot, whisk together the sugar, cornstarch, vinegar, and reserved beet liquid. Bring the mixture to a gentle boil. Whisking constantly, cook for about 1 minute or until the sauce has thickened slightly. Remove from the heat.
Stir in the sliced beets and cook just long enough to heat through. Season with salt and pepper.