Combine sesame oil, tamari soy sauce, balsamic vinegar, sriracha, and brown sugar in a bowl, then whisk to combine. Place ribeye in a large glass container, then pour marinade on top. Cover tightly and refrigerate.
When you're ready to begin cooking, take the steak out of the refrigerator and allow it to come to room temperature. We usually let it sit out.
Set a large cast iron skillet over high heat. After about 5 minutes, add a couple tablespoons of a neutral oil, such as grapeseed, avocado, or canola oil.
Carefully lift the ribeye steak out of the marinade, allowing excess to drip off. Place gently in the hot skillet. Watch out for any oil that may splatter!
Using tongs, carefully flip the steak over every 30 seconds, until a deep crust forms. Cook. We recommend using a meat thermometer to ensure proper doneness.
Once ribeye has reached desired temperature, use tongs to transfer steak to a cutting board. Allow meat to rest for 10 minutes before slicing.
Use a sharp knife to slice the ribeye into pieces about ¼-½" thick. Garnish with sesame seeds and scallions, then serve immediately.