Sprinkle steak with 1 Tbsp of cornstarch, 1/2 tsp of salt and 1/2 tsp black pepper. Toss to lightly coat.
Heat 1 Tbsp vegetable oil in a large 12 inch skillet over medium high heat. To the pan add the bell peppers and onion. Season with remaining 1/2 tsp salt and 1/2 tsp black pepper. Cook for about 3-4 minutes or just until crisp tender and beginning to brown.
Add the garlic and ginger to the pan cooking for 1 minute longer. Remove from the skillet to a platter and keep warm.
Add remaining 2 Tbsp oil and sesame oil to the skillet. Add the steak strips in a single layer to the pan.
Cook in batches over medium-high heat for 1-2 minutes. Adjust cooking time depending on the thickness of the steak, adding additional oil as needed. Remove using a slotted spoon to the same platter with the bell peppers and onion.
Meanwhile, in a small bowl whisk together the remaining 3 Tbsp cornstarch with soy sauce, beef stock, brown sugar, rice wine vinegar and crushed red pepper flakes.
Once all of the steak has cooked, add bell peppers and onion back to the pan. Pour the sauce over the steak and pepper mixture and bring to a simmer. Cook for 2-3 minutes over medium-high heat stirring constantly to coat and the sauce has thickened.
Serve immediately over white rice garnished with sesame seeds and green onion.