Canned beet root. Comes whole and sliced (most common). If all you can get is fresh, you would have to peel it, and boil till almost tender to substitute for canned. (assuming the cooking time is not long enough to cook it) The juice in the can if included in the recipe could be substituted by the cooking liquid from above. If you are going to use the cooking liquid I would just cover the beets to use as little as possible so it picks up as much flavor as possible.
By the way the juice can be used for for coloring.