For the steak: Whisk together the vinegar, soy sauce, fresh ginger, mustard, black pepper, garlic, lime zest and juice and candied ginger in a large shallow dish. Place the steak in a flat layer in the marinade, then cover tightly with plastic wrap and marinate at room temperature for 15 minutes, flipping the steak halfway through.
For the street corn: Bring 8 cups of water, along with the sugar and salt, to a simmer in a medium pot and stir to dissolve. Once dissolved, bring the liquid to a boil. Drop the corn into the boiling water and allow to cook until slightly tenderized, 4 to 5 minutes. Remove from the water and drain on a kitchen towel.
Coat a cast-iron skillet with canola oil and heat over medium-high heat. Cut the corn in half crosswise and place directly on a fairly hot spot in the skillet. Cook, rotating the ears little by little so they char all around, for about 5 minutes.
Using the same skillet, heat additional canola oil over medium-high heat. Remove the steak from the marinade and shake off the extra liquid. Reserve any leftover marinade. When the skillet begins to smoke, add the steak and let sit untouched for 3 to 4 minutes. Turn the steak and scatter the butter pieces in the skillet. Move the steak around to coat in the butter. Cook for an additional 4 to 5 minutes on the second side. Let rest for 5 to 10 minutes.
Roll each corn cob on all sides in the Dried Chili Mayonnaise. Sprinkle the togarashi and cilantro on a large plate, then mix to blend. Roll each cob in the mixture, coating on all sides.
For the spinach: Prepare a large bowl with ice water and fit a colander inside the ice bath. Fill a medium pot with about 4 quarts water and bring to a simmer over medium heat. Add a generous handful of salt and bring to a boil. (The water should taste like sea water.) Add the spinach and cook only until it wilts, about 30 seconds. Drain from the water with a slotted spoon or strainer and plunge into the colander. Swirl the spinach in the cold water so it goes from hot to cold quickly and cools off. Shake off the excess liquid and drain in a towel-lined bowl. Roll the kitchen towel up and squeeze any excess water from the spinach. Transfer the spinach to a flat surface and sprinkle with the vinegar and bonito flakes.
To finish the steak: Wipe any excess grease from the skillet where the steaks were cooked and add the bean sprouts, tomatoes and extra marinade. Reduce over medium-high heat for about 3 minutes. Meanwhile, slice the steak against the grain and season to taste.
Add the corn and spinach to the bento box sections. Spoon some of the sauce with the tomatoes and bean sprouts on the section for the steak and top with sliced steak. Garnish the steak with additional sauce. Serve with extra Dried Chile Mayonnaise.
Dried Chile Mayonnaise:
Whisk together the sour cream and mayonnaise in a large bowl until smooth. Sprinkle the guajillo chile powder, ancho chile powder, coriander, salt, pepper, lime zest and juice over the mayonnaise. Whisk together, then taste and adjust seasoning. Set aside.