Combine the mirin, soy sauce, sesame oil, garlic and sriracha in a small bowl. Pour into a shallow dish just large enough to hold the flank steak. Add the steak and turn it several times to coat with the marinade. Cover and refrigerate for at least 4 hours (overnight is fine), turning once.
To make the chimichurri sauce, place the shallot and garlic in the work bowl of a food processor and pulse several times to mince. Add the ginger and pulse to combine. Add the cilantro, basil and mint and pulse until the herbs are finely chopped. Add the olive oil, vegetable oil, rice wine vinegar, fish (or soy) sauce and black pepper. Process until blended, taste and add salt if necessary. Transfer the mixture to a serving dish, cover and refrigerate until ready to serve.
Preheat a grill, grill pan or broiler on high heat. Cook the steak for 2-1/2 to 3 minutes per side for rare to medium-rare. Allow to rest for 5 to 7 minutes before slicing across the grain. Serve with Asian chimichurri sauce for dipping.
Tips for Making This Recipe
About mirin:
Mirin is a popular condiment used in Japanese cooking and a common ingredient in teriyaki sauce. It is essentially a sweetened rice wine, but varieties with very low alcohol content are easy to find in the ethnic grocery section of most supermarkets.