Method:
Put mushrooms in a bowl, cover with 1/2 cup boiling water and set aside to let soak 10 minutes.
Meanwhile, in a blender, combine cashews and 2 cups broth and blend until very smooth, about 1 minute. Pour cashew mixture into a large saucepan and add rutabagas, onion and remaining 4 cups broth. Remove mushrooms from the bowl with a slotted spoon, cut them into bite-size pieces and add them to the pot; carefully pour in the liquid the mushrooms soaked in, being sure to leave behind any grit that has settled in the bottom of the bowl.
Bring the soup to a boil over high heat, lower heat and simmer, uncovered, until rutabagas are tender, 15 to 20 minutes. Add bell pepper and lemon juice and simmer 3 minutes longer. Serve garnished with chives and lemon zest.