Preheat the oven to 375 degrees F.
Place the beets in the center of a large foil sheet. Drizzle with the oil and vinegar. Place a second large foil sheet on top of the beets, crimping the edges together to form an airtight pocket. Roast in the oven for 2 hours.
When the beets are ready, peel them using a kitchen towel. If roasted correctly, the skin should slide right off. Cut the beets into 1-inch cubes and allow them to chill.
Heat soybean oil in a large pan or deep-fryer to 375 degrees F.
Toss the beets in the cornstarch until covered, adding more cornstarch as needed. Fry in 2-cup batches, until the outer cornstarch layer becomes crispy.
Serve each batch with 1/4 cup mayonnaise. Garnish with shichimi togarashi and green onions.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.