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 bag or airtight container; pour marinade over steaks. Marinate steaks at room temperature for 20 minutes.
Heat a non-stick pan over medium heat.
Remove steaks from marinade and arrange in the hot pan; gently shake pan to avoid sticking. Cook approximately 1 1/2 minutes and flip. Cook other side until tuna is desired doneness, 1 to 2 more minutes. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C).