3 tablespoons mirin, (see Note)
2 tablespoons reduced-sodium soy sauce
2 teaspoons cornstarch
1 tablespoon canola oil
8 ounces flank steak, trimmed of fat and very thinly sliced against the grain (see Tip)
1 tablespoon chopped garlic
2 teaspoons chopped jalapeno pepper, or to taste
1 1/2 teaspoons chopped fresh ginger
4 cups mung bean sprouts
1 6-ounce bag baby spinach
1/4 cup chopped fresh cilantro
1 teaspoon toasted sesame oil
2 tablespoons toasted sesame seeds, (see Tip), optiona