Cook the soba noodles according to package directions, about 4 to 5 minutes. Drain and rinse under cold water, then set aside.

In a saucepan over medium heat, cook the sliced shiitakes with a pinch of salt until they release their liquid and begin to brown, about 3 minutes. Add the vegetable broth and soy sauce, bring to a simmer, and cook for 5 minutes. Remove from heat and whisk in the miso paste until dissolved.

Divide the spinach and cooked noodles between two bowls. Ladle the hot broth over them, then top with halved soft-boiled eggs if using, the green onions, and a drizzle of toasted sesame oil. Serve immediately.