Thinly slice the flank steak into ¼-inch thick slices, going against the grain. Season the meat evenly with salt and pepper.
Whisk the ingredients for the sauce together until the cornstarch is dissolved and set aside.
Heat a bit of oil in a large skillet over high heat. Add the steak and saute until just browned. Remove the meat from the pan.
Using the same pan, add the broccoli, peppers, and onion to the pan. Cook until the vegetables are just slightly tender. Add the garlic and cook for 30 seconds more.
Add the steak back to pan, along with the reserved sauce and simmer until thickened.