Start by sorting through the mung beans, tossing out any discolored or shriveled ones. Rinse the beans under cold water until the water runs clear, and then let them drain.
In a large pot, add the mung beans and water. Bring them to a boil over medium heat, and then skim off any froth that floats to the top.
Lower the heat, cover the pot, and continue cooking for around 45 to 50 minutes until the beans have softened and their skins have burst. If needed, add more water to maintain about 4 cups.
While the mung beans are cooking, heat oil in another pot over medium heat.
Add the onions and garlic, and cook until they're tender and aromatic. Then, add the chopped tomatoes, cooking and mashing them with the back of the spoon until they've softened. Add the fish sauce and cook for an additional 1 to 2 minutes.
Next, add the cooked mung beans (with the liquid) to the pot with the onion and tomato mixture. Bring everything to a boil, and then add the pork cracklings.
Cook until they've softened, and then season the stew with salt and pepper to your liking.
Finally, add in the fresh spinach, turn off the heat, and cover the pot for 1 to 2 minutes until the spinach is just wilted.
Serve hot and enjoy!