Cut the flank steak lengthwise into two pieces and slice it across the grain into ⅛-inch-thick strips. In a small bowl, whisk the soy sauce, 2 teaspoons of sesame oil, sugar, and cornstarch until dissolved, and pour this over the steak strips. Heat the remaining 2 teaspoons of sesame oil in a large skillet or wok over medium-high heat. Stir-fry the garlic, ginger, red pepper flakes, and scallions for 30 seconds. Add the bell pepper and corn and stir-fry for 2 minutes. Add the pea pods and stir-fry for 30 seconds. Remove the vegetables from the skillet and set them aside. Stir-fry the beef strips in two batches for 2 minutes on each side. Return the vegetables to the skillet and heat through. Serve over rice.