Marinade: Combine soy sauce, mustard, rice vinegar, sesame oil, honey, ginger, garlic, salt, and pepper.
Marinate Tuna: Coat tuna steaks in the marinade and refrigerate for 30 minutes.
Coat Tuna: Cover steaks with sesame seeds after marinating.
Heat Skillet: Heat coconut oil in a skillet over medium-high heat.
Sear Tuna: Sear each side of the tuna for 1-2 minutes, leaving the inside rare.
Rest and Serve: Let rest for 1 minute, slice thinly, garnish with green onions, and serve with leftover marinade as dipping sauce.