Stir-fry is a great weeknight dish because, after just a little prep, dinner is on the table in a jiffy. And although you might not think of them for a dish like this, sweetpotatoes are a natural. They cook up lightly browned yet tender, and are a delicious complement to the creamy coconut milk-laced sauce and silky cashews.
In a medium bowl, combine the coconut milk, broth, aji-mirin, soy sauce, salt, and red pepper flakes. Add the chicken and set aside for 15 minutes. Drain the chicken, saving the marinade. Stir the cornstarch mixture into the marinade and set aside.
In a large wok or skillet over high heat, warm the oil. Add the sweetpotatoes and cook, stirring occasionally, until lightly browned, about 3 minutes. Add the garlic and ginger and cook, stirring, until fragrant, about 30 seconds.
Add the chicken and cook, stirring occasionally, until the chicken is cooked through and the sweetpotatoes are tender, about 3 minutes. Add the marinade mixture and water chestnuts and cook, stirring occasionally, until the sauce comes to a boil and thickens, about 1 minute. Stir in the scallions and cashews.
Serve with rice.
Note: Look for aji-mirin in the ethnic or Asian section of most major supermarkets.