In a deep pot over medium heat, heat about 2 tablespoons of oil. Add saba bananas and cook, turning as needed, until lightly browned. Remove from pan and drain on paper towels.
Add potatoes and carrots and cook until lightly browned. Remove from pan and drain on paper towels.
Add Chorizo de bilbao and cook until lightly browned. Remove from pan and drain on paper towels.
Remove excess oil except for about 1 tablespoon. Add onions and garlic and cook until softened.
Add pork belly and cook until lightly browned.
Add tomatoes and cook, mashing with the back of a spoon, until softened and release juices.
Add fish sauce and cook for about 1 to 2 minutes.
Add water and bring to a boil, skimming scum that floats on top.
Lower heat, cover and simmer for about 20 to 30 minutes or until meat is tender.
Add pork and beans, potatoes, carrots, and Chorizo de bilbao. Continue to simmer until vegetables are tender.
Add green beans and saba bananas and cook for about 2 to 3 minutes.
Add pechay and cook for another minute.
Season with salt and pepper to taste. Serve hot.