Make the teriyaki marinade by combining the brown sugar, soy sauce, minced garlic, and minced ginger in a small saucepan. Heat over medium heat just until sugar is dissolved, about 2-3 minutes. Do not allow this marinade to get hot since it will be poured over the steaks. Remove 1 cup of marinade and set aside.
Add cornstarch and water to the remaining teriyaki sauce and whisk well to incorporate. Cook over medium-high heat until thickened and bubbly, stirring frequently. Remove from heat and set aside. This makes about ¾ cup of sauce for drizzling.
Place the steaks in a large baking dish. Pour the reserved 1 cup of marinade over the steaks. Poke a few holes in the steaks with a fork to tenderize the meat and allow the marinade to seep through (optional). Cover with plastic wrap and set in the refrigerator for 1 hour or up to 4 hours.
Remove the steaks from the refrigerator. Drain and discard the marinade.
Heat a large skillet over medium-high heat. Once hot, add 1 tablespoon vegetable oil and turn pan to coat. Add the steaks to the pan and cook for 3-5 minutes per side, or until your desired level of doneness. You may have to work in batches to avoid overcrowding. Simply transfer cooked steaks to a plate and cover with tented foil to keep warm.
Let the steaks rest for 5 minutes before slicing steaks across the grain into thin strips. Serve garnished with sesame seeds and green onions and drizzle with teriyaki sauce.
Notes
Storage: Skillet Teriyaki Steak can be stored in the refrigerator for 3-4 days in an airtight container. Allow steaks to fully cool prior to placing them in the refrigerator.To reheat leftovers: Gently reheat the steak in a skillet over low heat or in the microwave on half power for short intervals, until heated through.To freeze: Seal the steak in an airtight container or freezer bag for up to 3 months. Thaw in the refrigerator overnight prior to reheating.It’s best to thinly slice the steaks across the grain.If you would like a little spice, simply add a pinch or two of red pepper flakes or sriracha sauce to the steaks.While marinating the steaks in fresh homemade teriyaki sauce creates the most flavorful and tender steaks, if time is short, you can use store-bought teriyaki sauce in a pinch. Simply season steaks with salt and pepper, pan-sear them in a skillet, and then drizzle with teriyaki sauce.