To start, grab your flank steak and slice it thinly against the grain. Cutting against the grain helps tenderize the meat. Once sliced, place the steak into a large Ziploc bag and sprinkle the cornstarch over it. Seal the bag and shake it gently until each piece of steak is covered evenly in cornstarch. This coating helps create that satisfying crispiness during cooking.

In a mixing bowl, combine the soy sauce, brown sugar, water, cornstarch, red pepper flakes, sliced green onion, minced garlic, grated ginger, rice vinegar, and sesame oil. Give it a good stir until the sugar and cornstarch dissolve completely. It should look smooth and well-blended. This concoction will be your teriyaki sauce, and it’s going to steal the show.

Now, heat a large skillet over medium-high heat. Add 2-3 tablespoons of coconut oil and allow it to get hot. You'll know it's ready when a small drop of water sizzles upon contact.

Shake off any excess cornstarch from half of the beef and add it to the skillet in an even layer. Be careful; you don't want to overcrowd the pan. Let the meat cook undisturbed for about 4-5 minutes. This allows it to develop a nice sear. After that, use your tongs to flip the beef pieces and let them cook for an additional 2-3 minutes. Once done, transfer the beef to a plate.

Repeat the cooking process with the remaining beef and add a bit more oil if necessary. Once you've cooked the second batch of beef, you might need to lower the heat slightly. This will be the perfect time to add those delicious aromatics.

With the heat still on medium, toss the sliced green onions, minced garlic, and grated ginger into the pan. Sauté for 1-2 minutes, but keep an eye on it; we want to avoid burning them. Once fragrant, pour the teriyaki sauce mixture into the skillet while stirring to combine. Bring it to a simmer and let it cook for about 4-5 minutes, or until it has thickened to your liking.

Lastly, return your beef to the skillet, stirring to coat every slice in that glossy sauce. Once everything is well mixed, serve immediately. This dish is best hot and fresh, so pair it with steaming white rice or stir-fried veggies.