In a small bowl, combine the mushrooms and boiling water. Cover and let stand until the mushrooms are fully hydrated, 20 to 30 minutes. Remove the mushrooms; reserve the water. Trim off and discard the mushroom stems and thinly slice the caps.
In a large saucepan over medium-high, heat the oil until shimmering. Add the mushrooms and cook, stirring occasionally, until lightly browned, 4 to 6 minutes. Add the garlic and ginger; cook, stirring, until fragrant, 30 to 60 seconds. Add the broth, the mushroom liquid and 1 cup water. Bring to a simmer, scraping up any browned bits, then cover, reduce to medium and simmer, stirring occasionally, until the mushrooms are tender, about 30 minutes.
In a small bowl, whisk together the miso and ¼ cup of the hot broth until the miso is dissolved. Stir the miso mixture and kimchi into the broth. Return the soup to a simmer over medium-high, stirring occasionally, then remove from the heat. Serve sprinkled with the scallions and drizzled with sesame oil.
Optional garnish: Soft- or hard-cooked eggs, halved OR toasted sesame seeds OR both
