Thai Chicken with Cashews

1 bunch scallions
1 lb. boneless, skinless chicken thighs
1/2 tsp. salt
1/4 tsp. pepper
3 tbsp. vegetable oil
1 red bell pepper, chopped
4 garlic cloves, finely chopped
1 1/2 tbsp. finely chopped peeled fresh ginger (or 1 1/2 tsp. dried)
1/4 tsp. red pepper flakes
3/4 c. reduced-sodium chicken broth
1 1/2 tbsp. soy sauce
1 1/2 tsp. cornstarch
1 tsp. sugar
1/2 c. salted roasted whole cashews
Cooked white or brown rice for serving

Chop scallions, separating white and green parts. Pat chicken dry, then cut into 3/4-inch pieces and toss with salt and pepper. Heat wok or 12-inch heavy skillet (not nonstick) over moderately high heat until a drop of water evaporates immediately. Add oil, swirling to coat, then stir-fry chicken until golden in places and just cooked through, 4 to 5 minutes. Transfer to plate with slotted spoon. Add bell pepper, garlic, ginger, red pepper flakes, and scallion whites to wok and stir-fry until peppers are just tender, 5 to 6 minutes.

Stir together broth, soy sauce, cornstarch, and sugar, then stir mixture into vegetables in wok. Reduce heat and simmer, stirring occasionally, until thickened, 1 to 2 minutes. Stir in cashews, scallion greens, and chicken, along with any juices accumulated on the plate. Serve with rice.