Rinse fish under cold running water and drain well. Pat dry and season with salt and pepper to taste.
In a wide pan over medium heat, heat about 2-inch deep of oil. Carefully add fish and cook, turning as needed, until lightly browned. Remove from pan and drain on paper towels.
In a wide pot over medium heat, heat about 1/4 cup oil. Add potatoes and cook until lightly browned. Remove from pan and drain on paper towels.
Remove excess oil except for about 1 tablespoon. Add onions, garlic, and ginger and cook until softened
Add fish sauce and continue to cook, stirring occasionally, for about 30 seconds.
Add water and bring to a boil.
Add potatoes. Lower heat and simmer for about 3 to 5 minutes or until almost done.
Add fish and continue to cook for about 5 to 6 minutes or until fish is cooked through and potatoes are tender.
Add pechay and cabbage and continue to cook for about 1 to 2 minutes or until vegetables are tender yet crisp.
Add green onions and continue for about another 30 seconds.
Season with salt and pepper to taste. Serve hot.