Meanwhile, in a large skillet, heat the remaining 2 tablespoons of oil over moderately high heat until shimmering. Add the green beans and stir-fry until they begin to brown, about 3 minutes; season with salt and pepper. Add the water to the skillet, cover and steam until the beans are crisp-tender, about 5 minutes. Uncover and cook until the water has evaporated, about 3 minutes longer. Add the garlic and cook until fragrant, then add the fish sauce, oyster sauce, sugar and 1/2 teaspoon of pepper and stir to coat the beans.