Begin the recipe by preparing the sirloin steak. Grab some paper towels and use them to remove any excess moisture from the 1 ½ pounds sirloin steak. Cut it into even-sized pieces, about 1-inch cubes. Set aside.
Using a large bowl, prepare the marinade by adding 1 tablespoon soy sauce, 1 tablespoon mirin, 2 cloves minced garlic, ½ tablespoon minced ginger, and 1 teaspoon sesame oil.
Stir to combine before adding the prepared steak. Using tongs, toss to combine and fully coat the raw steak in the marinade.
Cover and place in the refrigerator for at least 30 minutes, but you can leave it for up to 8 hours. If you're in a rush, you can go ahead and cook it immediately, but it will not be as flavorful.
After the steak has had time to marinate, remove it from the refrigerator and allow it to come to room temperature.
Place a large skillet on the stovetop over medium-high heat and allow it to warm before adding the oil. Allow the 1 tablespoon vegetable oil to heat and add the marinated steak. Add 1 tablespoon butter.
Allow it to cook for 4 to 5 minutes, until the steak has browned on all sides. Carefully remove the steak from the pan and transfer it to a plate.
Serve warm topped with sesame seeds and some freshly chopped green onion. Enjoy!
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Notes
Cook the steak to your preferred level of doneness: Well done: 4-5 minutes, Medium: 3 minutes, Medium rare: 2 minutes.
Do not skip the marinade time for the best flavor. If you're short on time, allow it to marinate for at least 30 minutes until 1 hour. It really does make a difference.
Stir-fry or use tongs to move the steak around often as it cooks. The steak will cook quickly in the hot pan, so be sure to keep an eye on it.
Use a cutting board and sharp knife to cut the raw steak into small pieces that are even in size. You could also get the steak already cut and ready to go from the meat department.