Step 1Heat the oven to 400 degrees. Wash the beets and trim the stems to about 1 inch.
Step 2Roast the beets. Place the beets on a large piece of aluminum foil. Add the canola oil, peppercorns, garlic, thyme and salt, tossing to combine. Wrap the beets in foil and place on a baking sheet. Roast the beets until there is no resistance when they are pierced with a sharp knife, 25 to 50 minutes (timing will vary depending on the size of the beets). Remove from the oven and cool slightly.
Step 3When the beets are cool enough to handle, rub each warm beet with a paper towel to remove the skin. Trim away the stems and roots. Place the beets in a vacuum-pack bag and season with half of the olive oil and sherry vinegar and a pinch of salt, or to taste. Vacuum pack on high.
Step 4Refrigerate the beets for 8 to 12 hours to develop the flavors. Season with remaining olive oil and sherry vinegar just before serving.