Combine mirin, soy sauce and cornstarch in a small bowl.
Heat oil in a large nonstick skillet over medium-high heat. Spread steak out in the pan and cook until seared on one side, about 1 minute. Add garlic, jalapeno and ginger and cook, stirring, until fragrant, about 30 seconds. Add bean sprouts and spinach (the pan will be very full). Pour the mirin mixture into the pan and stir gently until the sauce thickens and the spinach is wilted, about 3 minutes. Stir in cilantro and sesame oil. Serve topped with sesame seeds (if using).