Prepare the flank steak by placing the steak on a cutting board and cover it with plastic wrap or wax paper. 

Use a meat tenderizer to tenderize the meat and pound it to a 1-inch thickness. 

Next, add the koster salt, pepper, garlic powder, brown sugar, and red pepper flakes to a bowl and mix until combined. 

Pour the spice mix over the flank steak and massage the spices into the meat. Be sure the spices cover the steak completely. 

Let the flank steak rest in the spices for 20-30 minutes or longer for tender and flavorful meat. 

While the meat is resting, prepare the pepper steak sauce by adding the soy sauce, maple syrup, rice vinegar, fresh ginger, and cornstarch to a mason jar. Cover the jar and shake until the cornstarch is dissolved. Set aside.

When the meat is ready, slice the meat into thin strips being sure to cut against the grain of the meat. 

Heat a large skillet over medium/high heat and add olive oil. 

When the olive oil is fragrant add the meat to the pan and cook the meat for 1-2 minutes on each side depending on how thin the meat is. There should be a bit of pink left in the meat when you take it out of the pan. 

Set the meat aside and leave all of the juices and left over oil in the pan. Turn the heat to medium heat and add the red and green peppers to the pan. Sauté the peppers for 4-5 minutes. 

Add the green onions to the peppers and sauté for an additional 2 minutes. 

Finally, add the steak pieces and the pepper steak sauce to the pan (be sure you shake the pepper sauce one last time to make sure that the cornstarch is dissolved). 

Stir the ingredients in the pan so that everything is coated in the sauce. 

The sauce will begin to thicken quickly. Heat the sauce and other ingredients until the sauce is thick and warm. 

Remove from heat and serve over white rice and sprinkle with sesame seeds.