In a pot over medium heat, heat oil. Add onions, garlic, and ginger and cook, stirring regularly, until softened.
Add fish sauce and cook for about 1 to 2 minutes.
Add water and bring to a boil.
Add frog legs. Lower heat, cover, and cook for about 10 minutes or until frog legs are tender.
Add chayote and cook for about 3 to 5 minutes or until tender.
Season with salt and pepper to taste.
Add spinach, turn off heat, and cover to allow spinach to cook in the residual heat. Serve hot.
Notes
You can soak the legs in milk for about 1 hour or salt water overnight before cooking to remove any impurities and whiten and tenderize the meat. For food safety, keep refrigerated.