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Add oil to pot and heat over medium-high heat. Add ginger and chilies and cook, stirring for 30 seconds. Add spices and cook, stirring, 30 seconds more. Add onion and cook, stirring to scrape up any browned bits, until softened and translucent, about 5 minutes. Add tomato paste and sweet potatoes and cook, stirring, 1 minute.
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Add diced tomatoes, stock or water, and a sprinkling of salt. Bring to a boil and cook 10-15 minutes, or until sweet potato is tender. Place peanut butter in a medium bowl, add one or two ladlefuls of hot soup, and stir until emulsified, then pour mixture back into soup.
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Reduce heat to a simmer, add spinach, cover and cook a few minutes, until spinach is cooked but still bright. Turn off heat and stir in lemon juice. Let cool slightly and taste; add salt if necessary. Serve in bowls with rice, garnished with green onion and chopped peanuts, if desired.