Sprinkle tuna collar with a small amount of salt and let it sit for 10 minutes. Boil water in a pot, blanch the tuna collar until the surface changes color, then drain.

Mix the broth ingredients (sake, mirin, sugar, soy sauce, water) in a pot. Add green onion and ginger, and heat over medium heat. Once boiling, add the tuna collar and pour the broth over it for about 3 minutes. Cover with a drop lid and a regular lid, then simmer on low heat for 30 minutes.