Prepare the mushrooms by removing the stems and cleaning with a damp cloth or paper towel. Then slice the mushrooms.
In a large saucepan, heat the olive oil over low heat. Add the mushrooms and a few pinches of salt, and stir. Let the mushrooms cook until soft, about 5 minutes, stirring only occasionally. Add the scallions, garlic, and ginger, and cook 1 minute more. Add the water and bring to a low simmer.
Into a small bowl, scoop some of the simmering water and add the miso paste, stirring until it dissolves, then add it back into your soup pot. Simmer over low heat for 15 minutes.
Bring a medium pot of water to a boil. Prepare the noodles according to the instructions on the package. Drain and rinse the soba noodles.
To the soup pot, add the cooked noodles, spinach, soy sauce, sesame oil, rice vinegar, and red pepper flakes. Stir until the spinach is wilted. Taste and adjust seasonings.
Note: Add a poached egg or tofu cubes to make it a meal.