To make the simple corn salsa, heat a heavy-based frying pan or griddle pan until screaming hot and heavily char the corn over high heat for around 10 minutes, turning occasionally. Season generously with sea salt and leave to cool slightly.
While the corn cooks and cools, finely chop the red onion, coriander, and chilli if using.
Also pat the tuna dry with kitchen roll/paper then drizzle and rub with cooking oil, followed by sea salt.
Turn the corn on the cob onto its end and, using a sharp knife, slice the kernels off. Mix with the red onion, coriander, chilli, and lime juice. Season with salt to taste. Set aside.
Slice the sub rolls and drizzle each side with a little olive oil. Place under a hot grill for 1-2 minutes, until lightly toasted. Drizzle Kewpie mayo over both halves, to taste.
Heat a saucepan large enough for the tuna over high heat for around 1 minute, then gently place the tuna into the pan and cook on each side for one minute.
Remove the tuna from the pan and season with freshly ground black pepper, then use a sharp knife to cut into 2-5mm slices.
To assemble the sandwich, lay the tuna slices on the bottom half of the sub roll, then top with a tablespoon or so of the corn salsa and a pinch of sesame seeds.