Rinse it and wipe dry with paper towels or tissues. Place in a plastic or glass container on the upper shelf of the fridge next to a wall and cover with a clingfilm.
Start cooking tuna when it is half-frozen to preserve the juiciness and soft texture.
Heat grapeseed oil in a medium non-stick sauté pan over medium heat for 3 minutes. Do not let the oil get hot to the point where it starts smoking or it will burn the sesame seeds before cooking the fish.
Do not let the oil get too cold, or the fish will overcook before any color is achieved. Start with only one tuna portion at a time to get familiar with the cooking process.
Set the sesame-crusted tuna loins in the oil. Tilt the sauté pan to create a reservoir of hot oil in the bottom edge of the pan, keeping the tuna loin close to the elevated edge of the pan, away from the oil.