Combine 1 cup broth and dried shiitakes in a small saucepan. Bring to a boil over medium-high heat. Cover, reduce heat to maintain a simmer and cook for 10 minutes. Strain the broth through a coffee filter (or a double layer of cheesecloth) to catch the grit, and squeeze the mushrooms to extract as much liquid as possible. Reserve the cooking liquid and chop the mushrooms.
Meanwhile, heat oil in a large pot over medium-high heat. Add onion and cook, stirring frequently, until starting to brown, 2 to 4 minutes. Reduce heat to medium and continue cooking, stirring often, until the onion is soft, 3 to 5 minutes. Stir in the remaining 5 cups broth, scraping up any browned bits. Cover and bring to a boil over high heat. Add ginger, jalapeños, curry paste, tamari, lime zest and juice, salt and the reserved mushroom-cooking liquid. Cover and return to a boil.
Reduce heat to medium and add coconut milk, tofu, fresh mushrooms and the soaked shiitakes; return to a simmer and cook, partially covered, until the mushrooms are tender, 3 to 5 minutes. Stir in spinach and cook until wilted, 2 to 3 minutes more. Serve garnished with cilantro, if desired.
Tips
To make ahead: Refrigerate for up to 5 days.
