Working over a bowl to collect the juice, cut the grapefruit into supremes (see post for detailed instructions); place the fruit segments in one bowl and keep the juice in a separate bowl. Place the supremes in the refrigerator until ready to use.
To the grapefruit juice, add the minced ginger, garlic, green onion, coconut aminos, rice vinegar and salt; set aside.
Pat the tuna steak real dry and sprinkle it on both sides with a little bit of salt. Melt a little bit of healthy cooking fat in a heavy skillet set over really high heat, and wait until the pan gets scorching hot; when that happens, carefully place the tuna in the pan and let it sear for about 1 minute per side, or until a nice golden crust forms and the meat turns opaque about 1/4" up the side. Be careful not to overcook the fish, you still want to see a nice wide pink "ribbon" in the center of the steak when you look at it from the side.
Remove the fish to a container that's just big enough to accommodate it and pour the reserved marinade over it. Cover and place in the refrigerator to marinate overnight. Flip the tuna once or twice while it's marinating.
When you are ready to serve your tuna, remove it from the marinade and slice it into roughly 1/4" thick slices.
Serve with the grapefruit supremes; garnish with a handful of green sprouts, a pinch of sesame seeds and use marinade as a dipping sauce, if desired.
Since the marinade hasn't been used to marinate raw meat, it's perfectly safe to use as a dipping sauce.