In a small bowl, mix the mirin, soy sauce and cornstarch. Set aside.
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 the garlic, jalapeno and ginger and cook until fragrant, about 30 seconds. Add the spinach and the mirin mixture to the pan and stir until the sauce thickens and the spinach is wilted, about 3 minutes. Stir in cilantro and sesame oil, and remove from heat.
Spoon 1/2 cup of rice into a bowl or onto a plate and top with the stir-fry and enjoy!