In a small bowl, mix the mirin, soy sauce, and cornstarch.
In a large nonstick skillet, heat the oil over medium-high heat. Place the steak onto the pan, and cook until one side is seared (about 1 minute), then flip.
Add the garlic, jalapeno and ginger to the skillet. Cook for 30 seconds or until the mixture becomes fragrant. Add the bean sprouts and spinach to the skillet.
Pour the mirin and soy sauce mixture over the steak, and gently stir. After the sauce thickens and the spinach wilts (about 3 minutes), add cilantro and sesame oil.
Top with sesame seeds, and serve immediately.
