Chicken and Cashew Stir-Fry

1 pound skinless boneless chicken thighs

1/2 teaspoon salt

1/4 teaspoon black pepper

3 tablespoons vegetable oil

1 red bell pepper , chopped

4 garlic cloves, finely chopped

1 1/2 tablespoons finely chopped peeled fresh ginger

1/4 teaspoon dried hot red- pepper flakes

3/4 cup reduced-sodium chicken broth

1 1/2 tablespoons soy sauce

1 1/2 teaspoons cornstarch

1 teaspoon sugar

1/2 cup salted roasted whole cashew s