In a medium-large pot, heat the oil over medium heat. Add the mushrooms, white scallion slices, garlic, and ginger. Season with salt. Sauté, stirring occasionally, until mushrooms are tender, 6-7 minutes.

Add the vegetable broth and three cups water and bring to a boil. Add the noodles and reduce to a simmer. Add the spinach and wakame flakes and cook until the spinach is just wilted (but still vibrant green) and the wakame is rehydrated (it rehydrates fast). Add the lime juice, tamari, and a drizzle of sesame oil. Taste and adjust seasonings if necessary. Top with scallion greens, red pepper flakes or sriracha, and cilantro (if using).