We always use pork belly for yakiniku (grill or griddle cooked meat), since it lets us eat a ton of vegetables with it. We always use this 'green onion sauce' when we have grilled meat.
I liked the green onion topping served at a yakiniku restuarant, so I tried copying it. I make it with the white part of a Japanese leek sometimes too.
Buy a block of pork belly and cut it into 5 mm thick slices. I think it's most tasty this way.
Enjoy Japanese food around the world.