Step 1: Use a sharp knife to slice the tuna into thin pieces. Lay on a serving plate and season with salt (if using) and freshly ground black pepper. Keep covered in the fridge until needed.
Step 2: Toast the sesame seeds in a dry pan over moderate heat until golden. Remove and set aside.
Step 3: Heat a little vegetable oil in a pan and add the ginger, frying until slightly caramelised. Remove the ginger and set aside on a tray lined with kitchen paper. Do the same with the garlic, frying until slightly caramelised and draining on kitchen paper.
Step 4: Wipe the pan clean and add 30g vegetable oil and sesame oil. Once the oils are sufficiently warmed, pour over the seasoned tuna.
Step 5: Whisk the lemon juice, soy sauce and chilli together in a bowl. Pour over the tuna, then add the toasted sesame seeds, fried ginger and fried garlic.
Serve garnished with the Chinese onion and alfalfa sprouts.