In a wok, heat the oil until shimmering. Add the cashews and stir-fry over moderately high heat until golden, about 2 minutes. Using a slotted spoon, transfer the cashews to paper towels. Add the green beans to the hot oil and stir-fry until crisp-tender, about 4 minutes. Add the garlic, black beans and crushed red pepper and cook until the garlic is light golden, about 1 minute. Add the soy sauce and cook, stirring, until it coats the beans, about 1 minute. Remove from the heat and stir in the sesame oil. Transfer to plates and serve.