For make-ahead convenience, you can marinate the flank steak up to 24 hours. The spicy ginger marinade will permeate the meat for a more intense flavor.
1. In a bowl, stir soy sauce, sesame oil, rice vinegar, brown sugar, ginger, red pepper flakes and oyster sauce.
2. Place steak in a large resealable plastic bag; add half of the marinade; reserve remaining half. Refrigerate for 2 hours.
3. Heat broiler. In a medium-size skillet, cook beans and 1/2 cup water, covered, over medium-high heat for 5 minutes. Remove cover, add peppers and cook for 5 minutes or until water cooks off.
4. Meanwhile, coat a broiler pan with nonstick spray. Remove steak from marinade (and discard marinade). Broil steak 4 minutes per side or until instant-read thermometer reads 155 degrees F. Place on a cutting board to rest.
5. Stir cornstarch mixture into reserved marinade; add to beans along with cashews and cook 2 to 3 minutes or until thickened. Slice steak and serve with beans.