Toss the beets with the oil and vinegar in a large bowl until well coated. Line a baking sheet with aluminum foil to prevent leaks during cooking. Place the beets on the baking sheet and cover with aluminum foil to create a tight seal, allowing the beets to steam. Roast the beets until tender but still maintaining a bite, 3 hours.
Turn off the oven. Allow the beets to finish steaming for easier peeling. Peel the beets using a towel, making sure to wear gloves to prevent staining. Cut the beets into 1/2-inch cubes and place in a bowl.
Preheat oil in a deep-fryer or a deep pot to 350 degrees F.
Pour the cornstarch over the beets and toss until well coated. Place the beets in a sieve set over a metal bowl and shake to remove excess cornstarch. Transfer the beets to a fryer basket and fry until crispy and piping hot. When done, shake off excess oil and add beets to a clean metal bowl.
Season the beets with salt, then arrange on three serving dishes. Add a tablespoon of Japanese mayonnaise to each dish and sprinkle a little togarashi over the mayonnaise and the beets. Garnish with green onions.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.