Heat a wok over a high heat and add the oil, swirling to coat the sides. Stir-fry the chillies for 10 seconds until they begin to darken. Add the beans and garlic, and let them sizzle for 30 seconds until fragrant. Add the squash pieces and stir them around the wok to pick up all the fragrant oil, then pour in the stock or water and bring to a boil. Season with the sugar and salt, then cover, reduce the heat to maintain a gentle simmer, and cook for 4-5 mins or until the squash has absorbed most of the liquid and is tender enough to pierce with a chopstick.