1. Preheat the oven to 350°F (175°C).
2. Trim the tops and the bottoms of the beets and scrub them clean under running water. Place in the center of a large sheet of foil. Drizzle with the 1 tablespoon regular olive oil, season with salt and pepper, and pull the corners of the foil up, pinching the edges together to create a sealed tent around the beets. Make sure to leave as much room above the beets as possible so they can steam. Cook for 50 minutes to 1 hour, until you can slide a paring knife to the center easily. Remove from the oven and let cool.
3. Rub the beets gently to remove the skin.* Holding the beet on its side, slice into paper-thin rounds using a very sharp knife or a mandolin. Arrange the beet slices on a large flat platter as you go, overlapping one another on the edges in a fish scale pattern. After one layer is complete, sprinkle 1 tablespoon of tarragon leaves over the beets, then continue to layer the paper-thin rounds on top.
4. Drizzle the extra-virgin olive oil and sprinkle the vinegar over the beets.
5. Sprinkle with the sumac and tarragon. Season with salt and pepper and serve.
*Some beets will be harder to peel than others. I personally don’t mind beets skinned especially when the vegetable is shaved this thin. If you’re looking to save time, rub the beet and peel off the big chunks as much as you can and leave the parts that are more stubborn.