Stir together 3/4 cup soy sauce, sherry, sugar, 1 1/2 tablespoons garlic, 1 tablespoon sesame oil, and red pepper flakes in a 13 by 9 by 2-inch baking dish until sugar is dissolved, then add steaks, turning to coat. Marinate steaks, turning once, 1 hour.
Prepare grill for cooking.
Stir together cilantro, vegetable oil, lime juice, remaining 3 tablespoons soy sauce, remaining 1 1/2 teaspoons garlic, remaining 1/2 teaspoon sesame oil, and chile if using.
When fire is medium-hot (you can hold your hand 5 inches above rack 3 to 4 seconds), grill steaks on a lightly oiled rack 3 to 4 minutes on each side for medium-rare. Let steaks stand on a cutting board 5 minutes, then top with some sauce and cut crosswise into 3/4-inch-thick slices. Serve remaining sauce on the side.