Peel and dice the sweet potatoes and cook until soft.
Dice the onion, beat the lemongrass with the back of the knife and cut into 2-3 pieces so that it can be fished out of the soup at the end.
Heat the oil in a saucepan and sauté the onions, lemongrass and curry paste. Add the cooked potatoes. Pour in the lemon juice, soy milk and coconut milk and puree.