Separate the green onion into white and green parts. Mince the white part.

Pierce the pork belly with a fork. Place the pork, green onion green parts, and marinade ingredients (sake, salt, ginger, garlic) into a resealable bag. Massage and refrigerate for 1 hour.

Place the marinated pork in a microwave-safe dish, cover loosely with plastic wrap, and microwave at 600W for 4 minutes. Flip the pork, cover again, and microwave for another 4 minutes. Let it cool with the wrap on. Slice the pork and reserve 1 tbsp of meat juices for the sauce.

Mix minced green onion, reserved meat juices, and sauce ingredients (salt, pepper, lemon juice, sesame oil, sesame seeds) in a bowl to create the negi shio sauce.