Place shoyu, brown and white sugars, ginger, sherry, oil, green onions, garlic, white pepper and 3/4 gallon water in a large pot and bring to a rolling boil. Let cool, then strain out the solids so you are left with just teriyaki sauce. Place the ahi in a large bowl and cover with the teriyaki marinade (save remaining marinade for another use). Let marinade, refrigerated, 4 to 6 hours.
Preheat a grill for cooking at medium-high heat. Oil tuna and sprinkle with salt and pepper, then grill to desired doneness, 2 1/2 minutes per side for medium-rare.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.