Generously salt and pepper the steaks. Sear in a very hot cast-iron skillet, or another heavy-bottomed skillet, over very high heat for 45 seconds on each side.
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.
Seal using a vacuum sealer or the displacement method and place into a 130°F water bath for 4 hours.
Remove steaks from the bath and place them on the counter to rest. Reserve any juices that accumulated in the bag.
For the Dressing
Heat the sesame oil in a medium pot over medium heat, sauté onions until softened (about 5 minutes).
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.
Return the sauce to medium heat and add grated garlic and tobanjan.
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).
Remove from heat and mix in sesame seeds. Allow sauce to cool.
For the Salad
Add spring mix to the bowls. Slice the steak into thin pieces and place it on top. Add shredded carrots and daikon to the bowls and top with scallions. Dress with the teriyaki sauce with more on the side in case people want more.
Steak and sauce can be prepared up to 2 days ahead. Just assemble before you serve. If you want to add more to the salad: sliced cucumber and edamame work well too.
Keyword bowl, salad, sous vide, spicy, steak, teriyaki