Preheat oven to 450°F. Line a sheet pan with foil. Roast bones on sheet pan for 25-30 minutes.
Heat oil in large stock pot (mine is 8 quarts). Add garlic, ginger, bonito flakes, mushroom stems, and green onions. Saute 5 minutes or until fragrant. Add in bones and drippings from the pan.
Deglaze pan with sake. Simmer until liquid has reduced by half.
Add soy sauce and rice vinegar. Fill stock pot with water. Simmer for at least 8 hours.
Remove bones from broth. Strain broth with fine mesh strainer. Serve hot. Freeze leftovers in zippered bag.