Soba Soup with Spinach

2 tablespoons vegetable oil
12 ounces shiitake mushrooms (stems removed), caps thinly sliced (I used 14 ounces of tofu instead)
4 scallions, white and green parts separated and thinly sliced
1 garlic clove, minced
1 tablespoon peeled and minced fresh ginger
Coarse salt
2 cans (14.5 ounces each) reduced-sodium chicken broth
1/2 package (4.4 ounces) soba (Japanese buckwheat noodles)
1 bunch flat-leaf spinach, torn
2 tablespoons fresh lime juice
1 tablespoon soy sauce

In a large saucepan, heat oil over medium. Add mushrooms, scallion whites, garlic, and ginger; season with salt. Cook, stirring occasionally, until mushrooms are tender, 6 minutes.
Add broth and 3 cups water; bring to a boil. Add soba; reduce to a simmer, and cook 5 minutes. Add spinach; cook just until tender, about 1 minute. Add lime juice and soy sauce. Serve topped with scallion greens.

Serving Size: makes 4 bowls of soup