In a bowl, soak sotanghon in warm water for about 1 to 2 minutes or until just softened. Drain well and set aside.
In a pot over medium heat, combine beans and water. Bring to a boil, skimming off the froth that accumulates and beans that float on top. Lower heat, cover and cook for about 1 hour or until beans are softened, and skins have burst.
In another pot over medium heat, heat oil. Add garlic and onions and cook until tender and aromatic.
Add tomatoes and cook, mashing with the back of a spoon, for about 3 to 5 minutes or until softened and release juice.
Add shrimp and cook for about 1 to 2 minutes or until color changes to pink.
Add fish sauce and continue to cook for about 1 to 2 minutes.
Add cooked mung bean including liquid. Bring to a boil.
Add sotanghon and cook for 3 to 5 minutes or until noodles are translucent. Season with additional salt and pepper to taste.
Add spinach, turn off heat and cover pot for about 1 to 2 minutes or until spinach are just wilted. Serve hot.