Pour the teriyaki sauce into a shallow dish or bowl and place tuna steaks in the sauce to marinate. Cover and allow to marinate for about 45 minutes at room temperature. The steaks will cook more evenly if they are not cold.
Heat olive oil in a large frypan over medium heat. Place tuna steaks in the hot oil and pour in the teriyaki sauce. Squeeze juice from the lemon over the steaks and sprinkle with sesame seeds. When the bottom is light brown, flip the steaks and cook until browned on the other side. The centre will still be slightly red. Season with sea salt and pepper.
Place the tuna steaks onto serving plates and garnish with grated carrots. Serve the pan drippings as a dipping sauce.