Combine soy sauce and dark sesame oil in a medium bowl. Remove 2 tablespoons of the mixture to a small dish and set aside.
Cut beef into thin strips. Add to sauce mixture in bowl. Toss to coat.
Heat a large skillet over medium-high heat until hot. Add ½ of beef and stir-fry for about 1 minute or until outside surface of beef is no longer pink. Do not overcook. Remove beef from pan and repeat with the remaining beef strips.
Combine vegetables and ¼ cup water in same skillet (don’t clean the skillet out before cooking vegetables). Cook over medium-high heat 4 to 5 minutes or until most of the liquid is evaporated and vegetables are hot, stirring occasionally.
Combine cornstarch mixture and reserved marinade. Add to vegetables. Cook and stir 1 minute or until thickened and bubbly. Add beef; heat through.
Serve over rice and sprinkle with cashews if desired.