Place the fish in a food processor and pulse until chopped into small pieces (do not puree). Transfer to a medium bowl and mix in the mayonnaise, garlic, lemon juice, hot sauce, green onion, cornstarch, salt and pepper. Place the breadcrumbs in a wide, shallow dish. Dampen your hands lightly with cold water. Shape the fish mixture into 12 small balls, setting the balls on the breadcrumbs as you go along. Sprinkle the tops and sides of the fish mixture with breadcrumbs. Now press and form the balls into 2" wide cakes.
Heat the oil in a large non-stick skillet set over medium heat. Add the halibut cakes and cook for 3 to 4 minutes per side, until golden brown and cooked through. Stuff a cake in each bun with tartar sauce, lettuce and tomatoes.