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Generously salt and pepper the steaks. Sear in a very hot cast iron skillet. or other heavy bottomed, skilled over very high heat for 45 seconds on each side.
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Place the steaks in a Food Saver or Ziploc bag in a single layer. If they are too big you can slice them to fit.
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Seal using a vacuum sealer or the displacement method and place into a 130°F water bath for 4 hours.
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Remove steaks from the bath and place them on the counter to rest. Reserve any juices that accumulated in the bag.
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For The Dressing
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Heat the sesame oil in a medium pot over medium heat, sauté onions until softened (about 5 minutes).
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Add the soy sauce, mirin, sake, the steak juices, sugar, ginger, and garlic to the pot. Bring to a boil and then reduce to a simmer. Cook for 15 minutes. Using a sieve, strain out the solids.
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Return the sauce to medium heat and add grated garlic and tobanjan.
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In a small bowl, combine 1 tablespoon cornstarch and 1 tablespoon water and stir until it takes on a milky texture. Add this to the pot and cook, stirring occasionally, until the sauce becomes shiny and slightly thickened (about 3 minutes).
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Remove from heat and mix in sesame seeds. Allow to cool.