Heat the oil in a dutch oven or large pot over medium-high heat.
Add the mushrooms, yellow onion, garlic, and ginger to the pot and cook while stirring for about 1 minute.
Add the water and soy sauce to the pot and bring to a boil.
Reduce the heat to low then cover the pot and simmer, stirring occasionally, for about 45 minutes.
Stir in the noodles, corn, 1 tsp salt, and ½ tsp black pepper then simmer, stirring often, until noodles are tender, 15 to 20 minutes.
Stir in the green onion then season the soup with additional salt & pepper to taste.
