Heat the oil in a large pot, add the onion, garlic, ginger and fry for 5 minutes.
Add the curry paste to the onion mixture and cook, stirring for 1 minute. Add the sweet potatoes, stir to coat with the onion mixture.
Mix the tomato paste and vegetable broth in the saucepan. Bring to a boil, cover and simmer for 15-20 minutes until the sweet potatoes are almost tender.
Add the mushrooms and spinach to the stew and cook for 5 minutes or until the vegetables are cooked through.
Add peanut butter. To take the edge off the curry, add coconut milk if desired.
Add coriander, season with salt and pepper to taste. Serve garnished with coriander sprigs.