Sort through the mung beans, discarding discolored or shriveled beans and any grit or dirt. Rinse 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 legumes 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 garlic and onions and cook until tender and aromatic.
Add pork and cook, stirring occasionally, until lightly browned.
Add tomatoes and cook, mashing with the back of the spoon, until softened and release juice.
Add fish sauce and continue to cook for about 1 to 2 minutes.
Add cooked mung bean including liquid. Bring to a boil. Cook until pork is tender and liquid is slightly reduced.
Add coconut milk. Lower heat, and simmer for about 3 to 5 minutes or until stew is thickened to desired consistency. Season with salt and pepper to taste.
Add spinach, turn off heat and cover pot for 1 to 2 minutes or until spinach are just wilted. Serve hot.