Heat 1 teaspoon of vegetable oil in a large skillet over medium-high heat. Add the red and green bell pepper strips and sauté for 3–4 minutes, or until they reach a crisp-tender consistency. Transfer the peppers to a plate.
Pour the remaining oil into the pan. Lightly season the flank steak slices with salt and pepper. Increase the heat to high and add the steak to the skillet. Cook for 5–6 minutes, stirring occasionally, until the beef is browned.
Incorporate the minced garlic and ginger into the skillet with the beef. Stir for about 30 seconds until aromatic.
Return the cooked bell peppers to the pan, mixing them with the steak.
In a small bowl, combine the soy sauce, sugar, cornstarch, and 1/4 cup of water. Whisk thoroughly to create a smooth sauce. Pour the mixture into the skillet and stir to coat the steak and peppers evenly.
Allow the sauce to simmer for 2–3 minutes, or until it thickens to the desired consistency. Remove from heat and serve immediately.