Clean and pat dry the tuna steaks.
Apply mustard paste to both sides of the steaks.
Spread the sesame seeds on a plate.
Roll the tuna steaks in the sesame seeds, ensuring an even coating.
Heat a skillet over medium heat.
Add coconut oil and let it melt.
Place the tuna steaks in the skillet.
Sear each side for about 45 seconds, creating a nice crust while keeping the interior rare.
Remove from heat and let the steaks rest.
In a small bowl, whisk together soy sauce, rice vinegar, sesame oil, honey, ginger, garlic, and red pepper flakes.
Stir in the green onions.
Serve this sauce with the seared tuna.