If you have a little extra time before dinner, put the steak in the freezer for about 20 minutes to help make it easier to slice thinly.
To toast sesame seeds, heat a small dry skillet over low heat. Add sesame seeds and stir constantly until golden and fragrant, about 2 minutes. Transfer to a small bowl and let cool.
Note: Mirin is a low-alcohol rice wine essential to Japanese cooking. Look for it in the Asian or gourmet-ingredients section of your supermarket. An equal portion of sherry or white wine with a pinch of sugar may be substituted for mirin.
People with celiac disease or gluten-sensitivity should use soy sauces that are labeled "gluten-free," as soy sauce may contain wheat or other gluten-containing sweeteners and flavors.