Step 1

Stir together soy sauce, cornstarch, and 1/3 cup water in a cup.

Step 2

Pat steak dry and sprinkle with salt and pepper. Heat 2 tablespoons oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking. Stir-fry half of steak until browned but still pink inside, about 4 minutes. Transfer to a bowl with a slotted spoon and repeat with remaining steak, adding 1 tablespoon oil to skillet.

Step 3

Add remaining tablespoon oil to skillet and stir-fry bell peppers, onion, and garlic, until onion is golden, 6 to 7 minutes. Stir in remaining 2 tablespoons water and cook, covered, 3 minutes. Return steak to skillet, then stir in cornstarch mixture. Bring to a boil and cook, stirring, 2 minutes.