Place the flank steak in the freezer for 10 to 15 minutes. Meanwhile, whisk together the oyster sauce, broth, sherry, brown sugar, sesame oil and cornstarch in a bowl; set aside. Remove the steak from the freezer. Slice the meat along the grain into 3 long pieces; slice again thinly against the grain into bite-size pieces. Season with salt and pepper. Heat 1 tablespoon of the oil in a large skillet or wok over high. Stir-fry the beef until lightly browned, about 3 minutes. Transfer the meat and accumulated juices to a clean bowl. Heat 1 tablespoon of the oil in the same skillet and add the broccoli, bell pepper and 1⁄3 cup of water. Cover and cook over medium heat, stirring occasionally, until the broccoli is bright green, about 3 to 5 minutes. Uncover and continue to cook until the liquid evaporates, about 2 minutes. Add the remaining tablespoon of oil, the garlic, green onions and ginger. Cook until fragrant, about 1 minute. Add the reserved oyster sauce mixture and beef, and toss until the beef and vegetables are well coated with the sauce. Serve over a bed of rice (or that cauliflower “fried rice”). Serves 4.