Pat the tuna steak dry with paper towels and season both sides evenly with sea salt and black pepper.
In a small glass jar or bowl, whisk together the coconut aminos, rice vinegar, toasted sesame oil, and ground ginger to create the dressing.
Chop the romaine lettuce, dice the tomato, and thinly slice the red onion, then toss them together in a large serving bowl.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Place the tuna in the hot skillet and sear for 1.5 to 2 minutes per side for a perfect medium-rare, ensuring a golden crust forms on the outside.
Remove the tuna from the pan, let it rest for 2 minutes, then slice it against the grain into thin strips.
Arrange the sliced tuna over the salad bed, drizzle with the prepared dressing, and serve immediately.