Mix soy sauce, olive oil, lemon juice, sesame oil, molasses, cayenne pepper, ginger, and garlic together in a bowl.
Place tuna steaks in a large resealable plastic bag or airtight container; pour marinade over steaks, coat with marinade, and seal. Marinate steaks at room temperature for 20 minutes.
Heat a nonstick pan over medium heat.
Remove steaks from marinade and discard any remaining marinade. Arrange steaks in the hot pan; gently shake the pan to avoid sticking. Cook for 1½ minutes. Flip and cook 1 minute more.