Fill a medium saucepan with water and set it over high heat for the noodles.

Set a large soup pot on the stove over medium-high heat.

Add the oil to the soup pot and heat it for about 30 seconds, until it looks loose and shiny.

Add the scallion whites and light greens, the sliced ginger, and the garlic to the soup pot. Cook for 1 to 2 minutes, stirring often, until fragrant and softened but not browned.

Add the sliced mushrooms to the soup pot. Cook for 5 to 6 minutes, stirring every minute or so, until the mushrooms give off their liquid and start to brown around the edges.

Add the diced tomatoes and all of their juices to the soup pot. Cook for 3 minutes, stirring and pressing on the tomato pieces with a spoon so they break down a little.

Pour the vegetable broth into the soup pot. Stir in the soy sauce and bring the broth to a boil over high heat.

Add the sliced napa cabbage to the boiling broth. Reduce the heat to medium and simmer for 4 to 5 minutes, until the cabbage is tender but still holds its shape.

When the water in the saucepan comes to a boil, add the rice noodles to the saucepan. Cook them according to the package, usually 2 to 4 minutes, until they are tender and no longer chalky in the center.

Drain the noodles in a colander. Rinse them briefly with warm water so they stop cooking and do not clump together.

Taste the broth in the soup pot. Add a pinch of the kosher salt if needed, and add a little more soy sauce if you want a deeper savory flavor.

Divide the cooked noodles among 4 bowls.