How to Make It
Soak the porcini in the boiling water until softened, about 20 minutes. Drain the porcini, reserving the liquid separately. Rinse the mushrooms to remove any grit, then coarsely chop them.
Melt the butter in a large deep skillet. Add the porcini and garlic and cook over moderate heat until the garlic is golden, about 3 minutes. Add the rosemary and cook for 1 minute. Pour in half of the porcini soaking liquid and simmer until reduced to 2 tablespoons, about 5 minutes. Add the spinach, 1 large handful at a time, tossing and letting it wilt slightly before adding more. Cook the spinach over moderate heat, stirring, until tender but still bright green, about 4 minutes. Season with salt and pepper and serve.