Heat vegetable oil in a large skillet or wok over medium-high heat.
Add sliced bell peppers and cook for 3-4 minutes until tender-crisp.
Add sliced flank steak to the pan and cook for 3-5 minutes until browned on all sides.
Add minced garlic and ginger, stir-fry for 30 seconds until fragrant.
In a small bowl, whisk together soy sauce, sugar, and cornstarch until smooth.
Pour sauce mixture over the steak and peppers.
Simmer for 2-3 minutes, stirring constantly, until sauce thickens and coats the meat.
Serve immediately over steamed rice.