Cut off all but 1.5-2 inches of beet greens and stems from the bulb. This is called the tap root, and when kept on during boiling it helps preserve some of the color from leaking out. Wash the greens and keep them for another use. Wash and scrub the beets.
Add beets to a pot and cover with water, a teaspoon of salt and a teaspoon of vinegar (which helps prevent more color loss as well.) Add more water when necessary to keep the water above the beets and cook until the beets can be pierced easily with the point of a knife (about 30-35 minutes.) Smaller beets can be removed sooner than the large ones.
When the beets are done, drain the boiling water and plunge them into ice water to chill. When they're completely cooled, cut off a thin slice from the root and the stem end and then slip off the skin. These cooked beets will keep well in the fridge for a few days.
To continue with the recipe, cut the cooked beets in half, and then into either a large dice or in semi-circle slices. Heat up the sugar, the cornstarch and the remaining vinegar in a small pot and whisk until thickened and bubbling, and add in the beets. Stir and cook the beets until the sauce becomes thick and tacky, and has totally dressed the beets. Serve warm.