Pat the ahi tuna steak dry with paper towels and season both sides with sea salt and black pepper.
Heat the toasted sesame oil in a non-stick skillet over high heat until shimmering.
Place the tuna in the hot skillet and sear for 1 to 2 minutes per side, ensuring the center remains rare and the outside is golden brown.
Remove the tuna from the pan and let it rest for a few minutes before slicing into thin strips.
In a serving bowl, combine the warm cooked brown rice and the riced cauliflower to create a high-volume base.
Arrange the sliced cucumber, radishes, edamame, and avocado over the rice mixture.
In a small jar, whisk together the tamari, fresh grated ginger, and lime juice until well combined.
Place the sliced seared tuna on top of the bowl, drizzle with the ginger-lime dressing, and garnish with black sesame seeds.