Sort through the mung beans, discarding discolored or shriveled beans and any grit or dirt. Rinse beans in cold, running water until water runs clear. Drain well.
In a large pot, combine mung beans and water. Over medium heat, bring to a boil skimming froth that floats on top.
Lower heat, cover, and continue to cook for about 45 to 50 minutes or until beans have softened and skins have burst. Add more water as needed to maintain about 4 cups.
In another pot over medium heat, heat oil. Add onions and garlic and cook until tender and aromatic.
Add fish sauce and continue to cook for about 1 to 2 minutes.
Add cooked mung beans, including liquid. Bring to a boil.
Add ampalaya and cook for about 1 to 2 minutes or until tender yet crisp.
Add tinapa flakes and continue to cook until just heated through.
Season with salt and pepper to taste.
Add spinach, turn off heat and cover pot for about 1 to 2 minutes or until spinach is just wilted. Serve hot.