In a small bowl, whisk together soy sauce, grated ginger, minced garlic, sesame oil, honey, and rice vinegar to make the marinade. If using, add red pepper flakes.
Place the tuna steaks in a shallow dish and pour half of the marinade over them. Let marinate for at least 10 minutes, flipping halfway through.
Heat vegetable oil in a large skillet over medium-high heat until shimmering.
Remove tuna from marinade, letting any excess drip off. Sear tuna steaks for 1-2 minutes per side for rare, or longer for desired doneness. Avoid overcooking.
While tuna is searing, pour the remaining marinade into the same skillet and bring to a simmer. Cook for 1-2 minutes until slightly thickened.
Slice the seared tuna and arrange on plates. Drizzle with the thickened sauce. Garnish with sesame seeds and chopped cilantro.
Notes
Serve immediately with rice, a salad, or your favorite side dish.