Heat the oil in or skillet over medium heat until hot and stir-fry 1/2 of the basil leaves for 2 to 3 minutes until crispy. Remove and drain on paper towels. Set aside.
In the same pan, heat the leftover oil over medium heat and stir-fry the garlic, shallots and chillies for 2 to 3 minutes. Add the beef and stir-fry for 2 to 3 minutes until just cooked. Add the green beans and ginger, and season with the fish sauce, oyster sauce and sugar. Continue to stir-fry for another 2 to 3 minutes until the green beans are tender and cooked. Stir in the remaining basil leaves and remove from the heat.
Transfer to a serving platter, garnish with cashew nuts and the reserved crispy fried basil leaves, and serve hot with steamed rice.