Slice the steak

• Pat the flank steak dry.
• Thinly slice the steak against the grain into strips about ¼ inch thick.
• Transfer the steak to a bowl.

Marinate the steak

• Add the soy sauce, minced garlic, oil, rice vinegar, cornstarch, black pepper, sugar and baking soda to the steak.
• Mix well so the steak is fully coated in the marinade.
• Cover the bowl and refrigerate.

Make the sauce

• In a separate bowl, combine the soy sauce, beef stock, rice vinegar, sugar, cornstarch and black pepper.
• Whisk well and set aside.

Sear the steak

• Heat a large skillet over medium-high heat and add the oil.
• Once the oil is hot and shimmering, add the steak in batches without overcrowding the skillet.
• Sear about 1 minute per side until browned, then remove and repeat with the remaining steak.

Sauté the vegetables

• Reduce the heat to medium and add more oil if needed.
• Add the chopped onion and bell peppers, sprinkle with a pinch of salt.
• Sauté until slightly softened.

Simmer the sauce and combine

• Pour the sauce into the skillet and stir.
• Simmer until the sauce thickens.
• Return the steak to the skillet and mix well until everything is combined and the steak is reheated.