Preheat oven to 400 degrees. Line a large piece of aluminum foil with parchment paper. Place beets in center of parchment paper. Drizzle with oil, and season with salt and pepper. Bring aluminum foil together to completely enclose beets. Place on a baking pan, and roast until tender when pierced with the tip of a sharp knife, about 1 hour.
Remove from oven, and let stand until cool enough to handle. Rub off skins, and discard. Trim root ends, thinly slice beets, and transfer to a platter.
Pour rice vinegar into a small bowl. Season with salt, pepper, and sugar. Whisk to combine. Top beets with mango, and drizzle with vinegar mixture. Garnish with chervil. Serve.