In a saucepan over medium heat, combine pork belly and water. Bring to a boil, skimming scum that floats on top.
Lower heat, cover, and simmer for about 7 to 10 minutes or until pork is tender but not falling apart. Add more water as needed to maintain about 1 cup.
Drain pork, reserving 1 cup liquid. Slice pork into ½-inch cubes.
In a wok or wide pan over high heat, heat oil. Add onions and garlic and cook until softened.
Add pork and cook until lightly browned.
Add vegetables and stir to combine. Cook for 1 to 2 minutes.
In a small bowl, combine the reserved liquid, oyster sauce, and cornstarch. Add to pan and gently stir to distribute.
Continue to cook, stirring occasionally, for about 3 to 5 minutes or until vegetables are tender yet crisp and sauce is slightly thickened.
Season with salt and pepper to taste. Serve hot.