How to Make It
Combine the porcini, Parmesan rind, and 10 cups water in a large pot and bring to a boil. Reduce heat and simmer for 15 minutes. Strain, discarding the solids and reserving the liquid. Stir in the soy sauce.
Heat the oil in a large pot over medium-high heat. Add the leeks, ¾ teaspoon salt, and ¼ teaspoon pepper. Cook, stirring occasionally, until tender, 4 to 6 minutes. Add the mixed mushrooms and cook, stirring occasionally, until the mushrooms are tender and starting to brown, 6 to 8 minutes. Add the reserved broth, cover, and bring to a simmer. Add the soba and cook until heated through.
Serve the soup topped with the chives.