Slice off and discard the veiny stub of meat from the tuna, then slice into rough 3cm chunks. Place the sesame seeds onto a tray, then add the tuna chunks, turning them over in the seeds so they’re nicely coated.
Heat a good drizzle of olive and sesame oil in a medium frying pan over a medium heat. Peel and thinly slice the garlic, then add most of it to the pan. Fry for 1 to 2 minutes, or until golden and crisp, then use a slotted spoon to transfer to a plate (don’t drain away the oil!).
Return the pan to the heat and allow to heat up again, then add the sesame-coated tuna to the garlicky oil. Sear on one side for 10 to 20 seconds. Using tongs, turn the tuna over and keep cooking until you’ve seared only four sides of each chunk. Transfer to a plate.
Peel and finely grate the ginger, then add to a bowl with the soy, 1 teaspoon sesame oil and the remaining sliced garlic. Add a squeeze of lemon juice, then mix well to combine. Have a taste and add a squeeze more lemon juice, if needed.
Cut the seared tuna into slices, roughly 1cm thick, then arrange on your plates. Drizzle over the Asian dressing, scatter the garlic chips on top and tear over the coriander leaves. Trim and finely slice the spring onion and chilli at an angle, scatter on top, then finish with a drizzle of olive oil.