- 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.