In and a pan, bring the pork, water and salt to a boil. Reduce the heat to medium and cook until the pork is tender, about 30 minutes. Remove the pork from the broth, reserving 2 cups of the broth. Let the pork cool slightly and then cut it into 1/2 –inch pieces. Set aside.
Heat the oil in a skillet over medium-high heat. Add the garlic and onion, and sauté for 1 minute. Add the tomato and sauté for another minute.
Add the pork and sauté until browned, about 2 minutes. Stir in the fish sauce and stir to combine.
Add the squash and reserved broth, and bring to a boil. Lower the heat to medium and cook, covered for 5 minutes.
Add the green beans and cook covered, stirring occasionally, until the green beans are crisp tender, about 5 minutes. Season with fish sauce, to taste.
Serve hot with steamed rice.