Mix warm water and 15g salt in a tray until dissolved. Place the tuna collar in the mixture to clean off debris. Remove and pat dry with paper towels. Wrap in paper towels, place in a tray, cover with plastic wrap, and defrost in the refrigerator for 2 hours.

Remove the paper towels and sprinkle sake evenly over the tuna collar. Let it sit for 10 minutes, then pat dry again.

Sprinkle 1 tsp salt evenly over the tuna collar.

Lightly grease the grill rack with salad oil. Place the tuna collar skin-side down and grill on medium heat for 10 minutes. Flip and grill for another 10 minutes until cooked through.